refactor(minimotorways): improve roundabout visuals and traffic flow

- Redesign roundabout rendering with concentric rings (outer border, road
  surface, inner border, central island) for clearer visual distinction
- Increase ROUNDABOUT_CAP from 2 to 4 to allow more cars simultaneously
- Add stuck detection (stuckMs) with FORCE_ADVANCE_MS (6s) timeout to
  prevent cars from remaining permanently blocked
- Implement "don't block the box" logic: cars won't enter bare junctions
  if their exit cell is occupied, reducing gridlock

public/src/games/minimotorways/MiniMotorwaysGame.js

@@ -511,10 +511,19 @@ export default class MiniMotorwaysGame extends Phaser.Scene {
     for (const [k, item] of sim.items) {
       const cx = cellCx(k); const cy = cellCy(k);
       if (item.type === 'roundabout') {
-        g.fillStyle(this.city.palette.land, 1);
-        g.fillCircle(cx, cy, CELL * 0.16);
-        g.lineStyle(5, darken(this.city.palette.roadEdge, 0.7), 1);
-        g.strokeCircle(cx, cy, CELL * 0.16);
+        const pal = this.city.palette;
+        // Outer ring border
+        g.fillStyle(darken(pal.roadEdge, 0.88), 1);
+        g.fillCircle(cx, cy, CELL * 0.34);
+        // Ring road surface (covers road cap circles drawn beneath)
+        g.fillStyle(pal.road, 1);
+        g.fillCircle(cx, cy, CELL * 0.31);
+        // Inner ring border
+        g.fillStyle(darken(pal.roadEdge, 0.82), 1);
+        g.fillCircle(cx, cy, CELL * 0.18);
+        // Central island
+        g.fillStyle(pal.land, 1);
+        g.fillCircle(cx, cy, CELL * 0.14);
       } else {
         g.fillStyle(0x2f2f38, 1);
         g.fillRoundedRect(cx - 9, cy - 14, 18, 28, 5);

public/src/games/minimotorways/MiniMotorwaysLogic.js

@@ -45,8 +45,9 @@ export const TUNE = {
   MOTORWAY_MS: 1200,           // real traversal time, ignores all traffic
   MOTORWAY_MIN_DIST: 4,        // portals must be at least this far apart
   CONGESTION_COST: 0.4,        // A* edge penalty per car on the target cell
+  FORCE_ADVANCE_MS: 6000,      // after this long stuck, car overrides canEnterCell
   LIGHT_PHASE_MS: 4000,
-  ROUNDABOUT_CAP: 2,
+  ROUNDABOUT_CAP: 4,
   DISPATCH_MS: 500,
   SECOND_CAR_WEEK: 2,          // houses gain a second car from this week on
   COLOR_UNLOCK_WEEKS: [0, 0, 3, 5, 8, 11],
@@ -859,7 +860,7 @@ export class Sim {
       id: this.nextId++, color: house.color, houseId: house.id,
       state: 'idle', path: null, pos: 0, x: xOf(house.k), y: yOf(house.k),
       heading: 0, dwellT: 0, cooldownT: 0, targetId: null,
-      needsReroute: false, mw: null, occK: null,
+      needsReroute: false, mw: null, occK: null, stuckMs: 0,
     };
     this.cars.push(car);
     house.carIds.push(car.id);
@@ -894,6 +895,7 @@ export class Sim {
         car.state = 'toPickup';
         car.path = path;
         car.pos = 0;
+        car.stuckMs = 0;
         car.targetId = building.id;
         car.needsReroute = false;
         car.mw = null;
@@ -918,6 +920,7 @@ export class Sim {
     car.cooldownT = TUNE.COOLDOWN_MS;
     car.path = null;
     car.pos = 0;
+    car.stuckMs = 0;
     car.mw = null;
     car.needsReroute = false;
     if (house) { car.x = xOf(house.k); car.y = yOf(house.k); }
@@ -936,6 +939,7 @@ export class Sim {
     car.state = 'toHome';
     car.path = path;
     car.pos = 0;
+    car.stuckMs = 0;
     car.mw = null;
     car.needsReroute = false;
     this.setCarOcc(car, path[0]);
@@ -981,6 +985,7 @@ export class Sim {
     }
     car.path = path;
     car.pos = 0;
+    car.stuckMs = 0;
     car.mw = null;
     this.setCarOcc(car, path[0]);
     this.syncCarXY(car);
@@ -998,7 +1003,18 @@ export class Sim {
       const axis = dy === 0 ? 0 : 1;
       return axis === this.lightPhase();
     }
-    return occ === 0;
+    // Bare junction: only enter if empty, and don't block the box —
+    // hold outside if our exit is itself a bare occupied junction.
+    if (occ > 0) return false;
+    if (car.path && car.path.length > 0) {
+      const curIdx = Math.round(car.pos);
+      if (curIdx + 2 < car.path.length) {
+        const exitK = car.path[curIdx + 2];
+        if (this.connCount(exitK) >= 3 && !this.items.has(exitK)
+            && (this.cellOcc.get(exitK) || 0) > 0) return false;
+      }
+    }
+    return true;
   }
 
   headwayLimit(car, want) {
@@ -1031,13 +1047,18 @@ export class Sim {
         this.setCarOcc(car, car.path[Math.round(car.pos)]);
       }
       this.syncCarXY(car);
+      car.stuckMs = 0;
       return;
     }
 
     const want = TUNE.CAR_SPEED * (dtMs / 1000);
     let allowed = this.headwayLimit(car, want);
-    if (allowed <= 0.0001) return;
+    if (allowed <= 0.0001) {
+      car.stuckMs = (car.stuckMs ?? 0) + dtMs;
+      return;
+    }
 
+    const prevPos = car.pos;
     const curIdx = Math.round(car.pos);
     const boundary = curIdx + 0.5;
     let target = car.pos + allowed;
@@ -1053,9 +1074,11 @@ export class Sim {
         car.mw = { fromIdx: curIdx, t: 0 };
         this.setCarOcc(car, null);
         this.syncCarXY(car);
+        car.stuckMs = 0;
         return;
       }
-      if (!this.canEnterCell(car, nextK, fromK)) target = boundary - 0.01;
+      const forceThrough = (car.stuckMs ?? 0) >= TUNE.FORCE_ADVANCE_MS;
+      if (!forceThrough && !this.canEnterCell(car, nextK, fromK)) target = boundary - 0.01;
     }
 
     car.pos = Math.min(target, car.path.length - 1);
@@ -1063,10 +1086,17 @@ export class Sim {
     this.setCarOcc(car, car.path[Math.min(occIdx, car.path.length - 1)]);
     this.syncCarXY(car);
 
+    if (car.pos > prevPos + 0.0001) {
+      car.stuckMs = 0;
+    } else {
+      car.stuckMs = (car.stuckMs ?? 0) + dtMs;
+    }
+
     if (car.pos >= car.path.length - 1 - 0.0001) {
       if (car.state === 'toPickup') {
         car.state = 'dwell';
         car.dwellT = TUNE.DWELL_MS;
+        car.stuckMs = 0;
       } else if (car.state === 'toHome') {
         this.parkAtHome(car);
         car.state = 'cooldown';