feat(superkart): add beach theme and AI rubber-banding

- Add beach theme artwork and register paths in artwork JSON
- Implement AI rubber-banding that scales AI top speed based on gap to player
- Configure engine classes with varying rubber-band strength (Cruiser most forgiving, Turbo least)
- Balance racer stats: boost Kona top speed, reduce heavy racer accel/handling
- Update verification script to validate new rubber-banding configuration

data/superkart-artwork.json

@@ -41,7 +41,7 @@
     "zanthor":   { "key": "superkart-kart-zanthor",   "path": "assets/images/superkart/racer-zanthor.png", "frameWidth": 64, "frameHeight": 64, "angles": 8 }
   },
   "themeSheets": {
-    "beach":     { "key": "superkart-theme-beach",     "path": null, "frameWidth": 64, "frameHeight": 64 },
+    "beach":     { "key": "superkart-theme-beach",     "path": "assets/images/superkart/theme-beach.png", "frameWidth": 64, "frameHeight": 64 },
     "volcano":   { "key": "superkart-theme-volcano",   "path": null, "frameWidth": 64, "frameHeight": 64 },
     "scrapyard": { "key": "superkart-theme-scrapyard", "path": null, "frameWidth": 64, "frameHeight": 64 },
     "swamp":     { "key": "superkart-theme-swamp",     "path": null, "frameWidth": 64, "frameHeight": 64 },
@@ -49,7 +49,7 @@
     "nightcity": { "key": "superkart-theme-nightcity", "path": null, "frameWidth": 64, "frameHeight": 64 }
   },
   "backdrops": {
-    "beach":     { "key": "superkart-backdrop-beach",     "path": null },
+    "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 },

data/superkart-racers.json

@@ -35,7 +35,7 @@
       "id": "kona",
       "archetype": "Featherweight",
       "stats": {
-        "topSpeed": 0.35,
+        "topSpeed": 0.45,
         "accel": 0.7,
         "handling": 0.95,
         "weight": 0.15,
@@ -96,8 +96,8 @@
       "archetype": "Speed Demon",
       "stats": {
         "topSpeed": 1.0,
-        "accel": 0.45,
-        "handling": 0.4,
+        "accel": 0.10,
+        "handling": 0.2,
         "weight": 0.55,
         "offroad": 0.3,
         "drift": 0.45

data/superkart-rules.json

@@ -6,9 +6,9 @@
     "After editing, run `node tools/verifySuperKart.js` to re-check invariants."
   ],
   "engineClasses": [
-    { "id": "cruiser", "label": "CRUISER CLASS", "desc": "Relaxed speeds, forgiving rivals.", "speedMult": 0.8, "aiSkillMult": 0.75 },
-    { "id": "racer",   "label": "RACER CLASS",   "desc": "The intended Super Kart experience.", "speedMult": 1.0, "aiSkillMult": 1.0 },
-    { "id": "turbo",   "label": "TURBO CLASS",   "desc": "Everything faster, rivals ruthless.", "speedMult": 1.15, "aiSkillMult": 1.25 }
+    { "id": "cruiser", "label": "CRUISER CLASS", "desc": "Relaxed speeds, forgiving rivals.", "speedMult": 0.8, "aiSkillMult": 0.75, "rubberBandMult": 1.75 },
+    { "id": "racer",   "label": "RACER CLASS",   "desc": "The intended Super Kart experience.", "speedMult": 1.0, "aiSkillMult": 1.0, "rubberBandMult": 1.0 },
+    { "id": "turbo",   "label": "TURBO CLASS",   "desc": "Everything faster, rivals ruthless.", "speedMult": 1.15, "aiSkillMult": 1.25, "rubberBandMult": 0.25 }
   ],
   "physics": {
     "kartRadius": 14,
@@ -25,6 +25,11 @@
     "driftTurnMult": 1.6,
     "driftSlipRate": 240,
     "driftGripSpread": 170,
+    "miniTurboMinMs": 500,
+    "miniTurboMaxMs": 1800,
+    "miniTurboBoostMs": 650,
+    "miniTurboBoostMultMin": 1.1,
+    "miniTurboBoostMultMax": 1.4,
     "slipDecay": 3.2,
     "hopMs": 260,
     "hopCooldownMs": 160,

src/games/superkart/SuperKartGame.js

@@ -713,8 +713,7 @@ export default class SuperKartGame extends Phaser.Scene {
     if (kart.spinMs > 0) {
       frame = Math.floor((this.raceState.timeMs / 60)) % KART_ROT_FRAMES;
     } else if (kart.drifting) {
-      frame = kart.driftDir > 0 ? 1 : KART_ROT_FRAMES - 1;
-      angle = kart.driftDir * 6;
+      angle = kart.driftDir * 14;
     } else if (inputs.steer !== 0 && hasLean) {
       frame = inputs.steer < 0 ? KART_ROT_FRAMES : KART_ROT_FRAMES + 1;
     } else if (inputs.steer !== 0) {

src/games/superkart/SuperKartLogic.js

@@ -91,6 +91,7 @@ function makeKart(index, racer, isPlayer, slot, physics) {
     airborne: false,
     drifting: false,
     driftDir: 0,
+    driftChargeMs: 0,   // mini-turbo charge accrued while committed to the slide
     prevHop: false,
     prevItem: false,
     surface: SURFACE.ROAD,
@@ -174,9 +175,26 @@ function emit(state, type, data) {
 
 function classSpeedMult(state) { return state.engineClass?.speedMult ?? 1; }
 
+// Gap-to-player rubber-banding: AI karts trailing the player get a top-speed
+// boost, karts ahead get held back, scaled to 0 at the player's own
+// progress and saturating at +/-rubberBandRange world units of gap. Engine
+// class scales the whole effect (rubberBandMult) — Cruiser leans on this
+// hard for forgiving rivals, Turbo barely uses it to stay ruthless.
+function rubberBandMultOf(state, kart) {
+  if (kart.isPlayer) return 1;
+  const player = state.karts[state.playerIndex];
+  if (!player?.isPlayer) return 1;
+  const { physics } = state;
+  const gain = physics.rubberBandGain * (state.engineClass?.rubberBandMult ?? 1);
+  if (gain <= 0) return 1;
+  const gap = player.progress - kart.progress; // + = kart trails the player
+  const t = Math.max(-1, Math.min(1, gap / physics.rubberBandRange));
+  return 1 + gain * t;
+}
+
 function effTopSpeed(state, kart) {
   const { physics } = state;
-  let mult = classSpeedMult(state);
+  let mult = classSpeedMult(state) * rubberBandMultOf(state, kart);
   mult *= 1 + kart.coins * physics.coinTopSpeedBonus;
   if (kart.starMs > 0) mult *= itemParams(state, 'overdrive').mult ?? 1.2;
   if (kart.empMs > 0) mult *= itemParams(state, 'emp').mult ?? 0.6;
@@ -201,6 +219,7 @@ function respawnAtCheckpoint(state, kart) {
   kart.velAngle = kart.heading;
   kart.speed = 0;
   kart.drifting = false;
+  kart.driftChargeMs = 0;
   kart.airborne = false;
   kart.hopMs = 0;
   kart.offroadMs = 0;
@@ -214,6 +233,7 @@ function startSpin(state, kart, cause) {
   kart.spinMs = state.physics.spinMs;
   kart.spinSpin = 0;
   kart.drifting = false;
+  kart.driftChargeMs = 0;
   const dropped = Math.min(kart.coins, state.physics.coinsLostOnHit);
   kart.coins -= dropped;
   emit(state, 'spin', { kart: kart.index, cause });
@@ -359,14 +379,30 @@ function stepKart(state, kart, inputs, dt) {
       if (inputs.hop && steer !== 0 && kart.speed > effTopSpeed(state, kart) * 0.3) {
         kart.drifting = true;
         kart.driftDir = Math.sign(steer);
+        kart.driftChargeMs = 0;
         emit(state, 'drift-start', { kart: kart.index });
       }
     }
   }
   if (kart.drifting) {
+    kart.driftChargeMs += STEP_MS;
     const top = effTopSpeed(state, kart);
-    if (!inputs.hop || kart.speed < top * 0.25 || (steer !== 0 && Math.sign(steer) !== kart.driftDir)) {
+    const speedDropped = kart.speed < top * 0.25;
+    const steerReversed = steer !== 0 && Math.sign(steer) !== kart.driftDir;
+    if (!inputs.hop || speedDropped || steerReversed) {
       kart.drifting = false;
+      // Mini-turbo: only a clean release (hop let go, not a stall/reversal
+      // cutting the slide short) with enough committed charge pays out —
+      // stronger the longer the slide, capped at miniTurboMaxMs.
+      if (!speedDropped && !steerReversed && kart.driftChargeMs >= physics.miniTurboMinMs) {
+        const span = Math.max(1, physics.miniTurboMaxMs - physics.miniTurboMinMs);
+        const t = Math.min(1, (kart.driftChargeMs - physics.miniTurboMinMs) / span);
+        kart.boostMs = physics.miniTurboBoostMs;
+        kart.boostMult = physics.miniTurboBoostMultMin
+          + (physics.miniTurboBoostMultMax - physics.miniTurboBoostMultMin) * t;
+        emit(state, 'boost', { kart: kart.index, pad: false });
+      }
+      kart.driftChargeMs = 0;
       emit(state, 'drift-end', { kart: kart.index });
     }
   }

tools/verifySuperKart.js

@@ -57,6 +57,8 @@ for (const key of statKeys) {
 
 check('3 engine classes', rules.engineClasses.length === 3);
 check('classes ordered by speed', rules.engineClasses.every((c, i, a) => i === 0 || c.speedMult > a[i - 1].speedMult));
+check('classes ordered by inverse rubber-band strength (cruiser most forgiving)',
+  rules.engineClasses.every((c, i, a) => i === 0 || c.rubberBandMult < a[i - 1].rubberBandMult));
 check('points table has 9 entries', rules.pointsTable.length === 9);
 check('points table descending', rules.pointsTable.every((p, i, a) => i === 0 || p <= a[i - 1]));
 for (const item of rules.items) {
@@ -68,7 +70,7 @@ for (let col = 0; col < 9; col += 1) {
   check(`weight column ${col + 1} sums > 0`, total > 0);
 }
 const physKeys = ['baseTopSpeed', 'baseAccel', 'turnRateBase', 'hopMs', 'offroadMultMin',
-  'boostPadMult', 'spinMs', 'gridSpacing', 'rouletteMs', 'rubberBandGain'];
+  'boostPadMult', 'spinMs', 'gridSpacing', 'rouletteMs', 'rubberBandGain', 'rubberBandRange'];
 for (const k of physKeys) check(`physics.${k} present`, typeof rules.physics[k] === 'number');
 
 check('3 cups', cups.cups.length === 3);