feat(civilization): add coinage/public works special builds + fix AI fallback

- Add Coinage (1 shield → 1 gold) and Public Works (2 shields → 1 food)
  special build options for cities, with remainder carry logic in shieldBox
- Update AI to fall back to producing gold or food instead of stockpiling
  redundant defenders when nothing else is available
- Fix mid-build detection to check build.type before comparing shield progress
- Add City Screen UI for special builds: display gain per turn, color-coded
  rows, and tooltips
- Only apply class-switch penalty when switching between unit/building
  production types
- Add verification tests for coinage payout, public works remainder carry,
  and AI special build fallback

feat(superkart): fix player kart scale + add missing backdrop images

- Compute PLAYER_KART_SCALE from MODE7 parameters instead of hardcoding,
  keeping player and AI karts consistent at the camera's follow distance
- Wire up previously null backdrops: volcano, scrapyard, swamp, speedway,
  nightcity

data/superkart-artwork.json

@@ -50,11 +50,11 @@
   },
   "backdrops": {
     "beach":     { "key": "superkart-backdrop-beach",     "path": "assets/images/superkart/backdrop-beach.png" },
-    "volcano":   { "key": "superkart-backdrop-volcano",   "path": null },
-    "scrapyard": { "key": "superkart-backdrop-scrapyard", "path": null },
-    "swamp":     { "key": "superkart-backdrop-swamp",     "path": null },
-    "speedway":  { "key": "superkart-backdrop-speedway",  "path": null },
-    "nightcity": { "key": "superkart-backdrop-nightcity", "path": null }
+    "volcano":   { "key": "superkart-backdrop-volcano",   "path": "assets/images/superkart/backdrop-volcano.png" },
+    "scrapyard": { "key": "superkart-backdrop-scrapyard", "path": "assets/images/superkart/backdrop-scrapyard.png" },
+    "swamp":     { "key": "superkart-backdrop-swamp",     "path": "assets/images/superkart/backdrop-swamp.png" },
+    "speedway":  { "key": "superkart-backdrop-speedway",  "path": "assets/images/superkart/backdrop-speedway.png" },
+    "nightcity": { "key": "superkart-backdrop-nightcity", "path": "assets/images/superkart/backdrop-nightcity.png" }
   },
   "itemSheet": { "key": "superkart-items", "path": null, "frameWidth": 48, "frameHeight": 48 }
 }

src/games/civilization/CivilizationAI.js

@@ -242,7 +242,8 @@ const DEVELOP_CHAIN = ['granary', 'library', 'marketplace', 'barracks', 'aqueduc
 function manageCityBuild(rules, state, civIdx, city, strategy) {
   const civ = state.civs[civIdx];
   const def = rules.units[city.build?.id];
-  const midBuild = city.shieldBox > 0 && city.shieldBox < buildCost(rules, city) * 0.9;
+  const midBuild = (city.build.type === 'unit' || city.build.type === 'building')
+    && city.shieldBox > 0 && city.shieldBox < buildCost(rules, city) * 0.9;
   const defenders = unitsAt(state, city.x, city.y)
     .filter((u) => u.civ === civIdx && rules.units[u.type].domain === 'land'
       && !rules.units[u.type].flags.includes('noncombat'));
@@ -335,8 +336,10 @@ function manageCityBuild(rules, state, civIdx, city, strategy) {
       return;
     }
   }
-  const fallback = bestDefender(rules, state, civ, city);
-  if (fallback) setBuild(rules, state, city, 'unit', fallback.id);
+  // Nothing left to build or develop — convert shields to gold or food
+  // instead of stockpiling redundant defenders.
+  const preferFood = yields.foodSurplus < 3;
+  setBuild(rules, state, city, preferFood ? 'food' : 'gold', preferFood ? 'publicworks' : 'coinage');
 }
 
 function bestDefender(rules, state, civ, city) {

src/games/civilization/CivilizationCityScreen.js

@@ -5,7 +5,7 @@ import { GAME_WIDTH, GAME_HEIGHT, COLORS } from '../../config.js';
 import { Button } from '../../ui/Button.js';
 import { Tooltip } from '../../ui/Tooltip.js';
 import * as Logic from './CivilizationLogic.js';
-import { describeUnitTooltip, describeBuildingTooltip } from './CivilizationTooltips.js';
+import { describeUnitTooltip, describeBuildingTooltip, describeSpecialBuildTooltip } from './CivilizationTooltips.js';
 
 const FONT = '"Julius Sans One"';
 
@@ -117,6 +117,16 @@ export function openCityScreen(scene, rules, state, city, onClose) {
 
     // --- Build picker (right column)
     const rx = px + 60;
+    const isSpecialBuild = city.build.type === 'gold' || city.build.type === 'food';
+    if (isSpecialBuild) {
+      const spec = Logic.SPECIAL_BUILDS[city.build.type];
+      const gain = Math.floor((city.shieldBox + y.shield) / spec.den) * spec.num;
+      const unitLabel = city.build.type === 'gold' ? 'gold' : 'food';
+      dynamic.add(scene.add.text(rx, top + 64,
+        `Producing: ${spec.name}   +${gain} ${unitLabel}/turn`, {
+          fontFamily: FONT, fontSize: '22px', color: COLORS.textHex,
+        }));
+    } else {
       const cost = Logic.buildCost(rules, city);
       const cur = city.build.type === 'unit' ? rules.units[city.build.id] : rules.buildings[city.build.id];
       const shownShields = Math.min(city.shieldBox, cost);
@@ -130,16 +140,19 @@ export function openCityScreen(scene, rules, state, city, onClose) {
         if (Logic.buyBuild(rules, state, city)) redraw();
       }, { width: 190, height: 46, fontSize: 18, variant: civ.gold >= buyPrice && !city.boughtThisTurn ? 'solid' : 'ghost' });
       dynamic.add(buyBtn);
+    }
     dynamic.add(scene.add.text(rx, top + 108, `Treasury: ${civ.gold} gold`, {
       fontFamily: FONT, fontSize: '19px', color: COLORS.goldHex,
     }));
 
-    // Choices: units then buildings, two columns of rows.
+    // Choices: Coinage/Public Works, then units, then buildings, two columns of rows.
+    const specialChoices = Object.entries(Logic.SPECIAL_BUILDS)
+      .map(([type, spec]) => ({ type, id: spec.id, label: spec.name }));
     const unitChoices = Logic.availableUnits(rules, state, civ, city)
       .map((u) => ({ type: 'unit', id: u.id, label: `${u.name}  (${u.cost})`, ruleObj: u }));
     const bldChoices = Logic.availableBuildings(rules, state, civ, city)
       .map((b) => ({ type: 'building', id: b.id, label: `${b.name}  (${b.cost})`, ruleObj: b }));
-    const choices = [...unitChoices, ...bldChoices];
+    const choices = [...specialChoices, ...unitChoices, ...bldChoices];
     const colW = 330;
     const rowH = 36;
     const perCol = 14; // leaves room below for the garrison strip
@@ -150,18 +163,20 @@ export function openCityScreen(scene, rules, state, city, onClose) {
       const rect = scene.add.rectangle(colX + colW / 2, rowY, colW, rowH - 4,
         active ? 0x3a3222 : 0x181510)
         .setStrokeStyle(1, active ? COLORS.gold : COLORS.muted, active ? 1 : 0.5);
+      const rowColor = ch.type === 'unit' ? '#cfe3ff' : (ch.type === 'building' ? '#ffe9bd' : '#c9f2c0');
       const txt = scene.add.text(colX + 10, rowY, ch.label, {
-        fontFamily: FONT, fontSize: '16px',
-        color: ch.type === 'unit' ? '#cfe3ff' : '#ffe9bd',
+        fontFamily: FONT, fontSize: '16px', color: rowColor,
       }).setOrigin(0, 0.5);
       rect.setInteractive({ useHandCursor: true });
       rect.on('pointerdown', () => {
         Logic.setBuild(rules, state, city, ch.type, ch.id);
         redraw();
       });
-      tooltip.attachTo(rect, () => (ch.type === 'unit'
-        ? describeUnitTooltip(rules, ch.ruleObj)
-        : describeBuildingTooltip(rules, ch.ruleObj)));
+      tooltip.attachTo(rect, () => {
+        if (ch.type === 'unit') return describeUnitTooltip(rules, ch.ruleObj);
+        if (ch.type === 'building') return describeBuildingTooltip(rules, ch.ruleObj);
+        return describeSpecialBuildTooltip(ch.type);
+      });
       dynamic.add(rect);
       dynamic.add(txt);
     });

src/games/civilization/CivilizationLogic.js

@@ -468,8 +468,22 @@ export function foundCity(rules, state, unit, name) {
   return city;
 }
 
+// Coinage/Public Works: convert shields straight into gold/food every turn
+// instead of banking them toward a unit/building. `den` lets food run at a
+// worse ratio than gold without losing shields — see processCity, which
+// reuses city.shieldBox as the carry register for the remainder.
+export const SPECIAL_BUILDS = {
+  gold: { id: 'coinage', name: 'Coinage', num: 1, den: 1 },
+  food: { id: 'publicworks', name: 'Public Works', num: 1, den: 2 },
+};
+
+function isProductionClass(type) {
+  return type === 'unit' || type === 'building';
+}
+
 export function setBuild(rules, state, city, type, id) {
-  if (city.build && city.build.type !== type && city.shieldBox > 0) {
+  if (city.build && city.build.type !== type && city.shieldBox > 0
+    && isProductionClass(city.build.type) && isProductionClass(type)) {
     city.shieldBox = Math.floor(city.shieldBox / 2); // Civ II class-switch penalty
   }
   city.build = { type, id };
@@ -574,12 +588,24 @@ function processCity(rules, state, city) {
     if (city.size <= 0) { destroyCity(rules, state, city); return; }
   }
 
-  // Shields. Capped at cost — a settler build stalled on city.size < 2
+  // Shields.
+  if (city.build.type === 'gold' || city.build.type === 'food') {
+    // Coinage/Public Works: shieldBox is repurposed as a carry register so
+    // the 2:1 food ratio never loses an odd shield across turns.
+    const spec = SPECIAL_BUILDS[city.build.type];
+    city.shieldBox += y.shield;
+    const gain = Math.floor(city.shieldBox / spec.den) * spec.num;
+    city.shieldBox -= Math.floor(gain / spec.num) * spec.den;
+    if (city.build.type === 'gold') civ.gold += gain;
+    else city.foodBox += gain;
+  } else {
+    // Capped at cost — a settler build stalled on city.size < 2
     // (see completeBuild) would otherwise keep banking shields turn after
     // turn with nowhere to go, showing e.g. "60/40 shields" and a negative
     // turn count once shields overshot the cost.
     city.shieldBox = Math.min(city.shieldBox + y.shield, buildCost(rules, city));
     if (city.shieldBox >= buildCost(rules, city)) completeBuild(rules, state, city);
+  }
   city.boughtThisTurn = false;
 
   // Economy.

src/games/civilization/CivilizationTooltips.js

@@ -1,5 +1,7 @@
 import { COLORS } from '../../config.js';
-import { cityById, cityAt, tileIndex, defenderStrength, unitsOnBoat, IMP } from './CivilizationLogic.js';
+import {
+  cityById, cityAt, tileIndex, defenderStrength, unitsOnBoat, IMP, SPECIAL_BUILDS,
+} from './CivilizationLogic.js';
 
 // Flavor/ability text for unit flags — data/civilization-rules.json has no
 // description field on units, so this is authored here. A few flags
@@ -101,6 +103,15 @@ export function describeBuildingTooltip(rules, building) {
   return { title: building.name, lines };
 }
 
+// Coinage/Public Works: no cost/upkeep to report, just what the conversion does.
+export function describeSpecialBuildTooltip(type) {
+  const spec = SPECIAL_BUILDS[type];
+  const text = type === 'gold'
+    ? 'Converts this city\'s shield production directly into gold, 1 shield = 1 gold, every turn.'
+    : 'Converts this city\'s shield production directly into food, 2 shields = 1 food, every turn.';
+  return { title: spec.name, lines: [{ text }] };
+}
+
 // Combat is always a duel against the tile's single strongest defender
 // (pickDefender), so enemy tooltips surface that unit's effective defense —
 // terrain, fortify, walls etc. included — instead of letting the raw stat

src/games/superkart/SuperKartGame.js

@@ -39,6 +39,11 @@ const D = {
 };
 const ITEM_FRAME = { bolt: 0, seeker: 1, oil: 2, turbo: 3, overdrive: 4, emp: 5, coins: 6 };
 const BOX_FRAME = 7;
+// The player kart is a screen-fixed sprite rather than a projected world
+// sprite, so its scale is pinned to what MODE7.project() would give an AI
+// kart sitting at the camera's follow distance — keeps both the same size
+// when a rival is drafting right alongside the player.
+const PLAYER_KART_SCALE = (MODE7.spriteBase / 64) * (MODE7.focal / MODE7.followDist);
 const COIN_FRAME = 8;
 
 export default class SuperKartGame extends Phaser.Scene {
@@ -520,7 +525,7 @@ export default class SuperKartGame extends Phaser.Scene {
     // Player kart, fixed at the bottom center like the SNES original.
     const pk = this.playerKartState();
     this.playerSprite = this.add.sprite(GAME_WIDTH / 2, GAME_HEIGHT * 0.75,
-      this.kartTexKey(pk.racer.id), 0).setDepth(D.playerKart).setScale(3.1);
+      this.kartTexKey(pk.racer.id), 0).setDepth(D.playerKart).setScale(PLAYER_KART_SCALE);
 
     this.debugG = this.add.graphics().setDepth(D.debug).setVisible(this.debugOn);
   }
@@ -721,7 +726,7 @@ export default class SuperKartGame extends Phaser.Scene {
     }
     s.setFrame(frame);
     s.setAngle(angle);
-    let scale = 3.1;
+    let scale = PLAYER_KART_SCALE;
     if (kart.empMs > 0) scale *= 0.55;
     if (kart.squashMs > 0) s.setScale(scale * 1.2, scale * 0.5);
     else s.setScale(scale);

tools/verifyCivilization.js

@@ -622,6 +622,62 @@ if (RULES) {
     check('revolution completes', civ.government === 'monarchy');
     check('cannot switch to unknown gov', !Logic.startRevolution(RULES, st, civ, 'democracy'));
   }
+
+  // Coinage: shields convert straight to gold, 1:1, every turn, no shieldBox growth.
+  {
+    const st = makeFlatState();
+    const settler = Logic.spawnUnit(RULES, st, 0, 'settlers', 5, 5, null);
+    const city = Logic.foundCity(RULES, st, settler);
+    Logic.setBuild(RULES, st, city, 'gold', 'coinage');
+    const civ = st.civs[0];
+    civ.gold = 0;
+    const y1 = Logic.cityYields(RULES, st, city);
+    Logic.beginCivTurn(RULES, st, 0);
+    const expected = y1.gold - y1.upkeep - y1.supportGold + y1.shield;
+    check('coinage pays exactly y.shield gold on top of normal trade income', civ.gold === expected);
+    check('coinage leaves shieldBox at 0', city.shieldBox === 0);
+    const goldAfter1 = civ.gold;
+    Logic.beginCivTurn(RULES, st, 0);
+    check('coinage keeps paying out turn after turn', civ.gold > goldAfter1);
+  }
+
+  // Public Works: shields convert to food at 2:1, with the odd shield carried
+  // in shieldBox instead of lost, and switching build types doesn't halve it.
+  {
+    const st = makeFlatState();
+    const settler = Logic.spawnUnit(RULES, st, 0, 'settlers', 5, 5, null);
+    const city = Logic.foundCity(RULES, st, settler);
+    Logic.setBuild(RULES, st, city, 'food', 'publicworks');
+    const y = Logic.cityYields(RULES, st, city);
+    const foodBoxBefore = city.foodBox;
+    Logic.beginCivTurn(RULES, st, 0);
+    const gain1 = Math.floor(y.shield / 2);
+    const remainder1 = y.shield - gain1 * 2; // 0 or 1, carried in shieldBox
+    check('public works food gain matches floor(shield/2)',
+      city.foodBox === foodBoxBefore + y.foodSurplus + gain1);
+    check('odd shield is carried in shieldBox rather than lost', city.shieldBox === remainder1);
+    const carry = city.shieldBox;
+    check('switching away from a special build does not halve the carry',
+      (() => { Logic.setBuild(RULES, st, city, 'building', 'granary'); return city.shieldBox === carry; })());
+  }
+
+  // AI fallback: an idle city with nothing left to build picks gold or food
+  // instead of endlessly stacking defenders.
+  {
+    const st = makeFlatState();
+    const settler = Logic.spawnUnit(RULES, st, 0, 'settlers', 5, 5, null);
+    const city = Logic.foundCity(RULES, st, settler);
+    st.civs[0].human = false;
+    // Give it a defender so the garrison branch doesn't fire, and mark every
+    // building/unit tier as already handled so develop/expand/war all pass.
+    Logic.spawnUnit(RULES, st, 0, 'phalanx', 5, 5, city.id);
+    for (const b of RULES.buildingList) city.buildings[b.id] = true;
+    delete city.buildings.palace; // keep palace semantics untouched
+    city.buildings.palace = true;
+    AI.runAITurn(RULES, st, 0);
+    check('idle AI city settles on Coinage or Public Works',
+      city.build.type === 'gold' || city.build.type === 'food');
+  }
 }
 
 // ---------------------------------------------------------------------------