feat(excitebike): add cool-zone speed boost and cooldown feedback

- Allow bikes to exceed normal turbo speed when holding through cool
  zones with turbo engaged, rewarding track knowledge
- Emit 'cooldown' event when entering cool zones to clear center msg
- Add coolZoneBoostSpeed tune constant (240) with design documentation
- Update physics invariant checks to use new speed cap

src/games/excitebike/ExcitebikeGame.js

@@ -707,6 +707,9 @@ export default class ExcitebikeGame extends Phaser.Scene {
           this.crt.pulse(0.6, 240);
         }
         break;
+      case 'cooldown':
+        if (ev.isPlayer) this.setCenterMsg('');
+        break;
       case 'lap':
         if (ev.isPlayer) playSound(this, SFX.EIGHTBIT_ACTIVATE);
         break;

src/games/excitebike/ExcitebikeLogic.js

@@ -61,6 +61,10 @@ export const TUNE = {
   heatRiseRate: 1 / 7,
   heatFallRate: 1 / 5,
   coolZoneRate: 1 / 1.2,
+  // Holding turbo through a cool zone also kicks the bike past its normal
+  // ceiling — a real reward for finding the chevrons, not just a way to
+  // dodge heat. Coasting through one gets no speed bonus, only the cooling.
+  coolZoneBoostSpeed: 240,
   overheatMs: 4000,
 
   // Lanes.
@@ -437,6 +441,7 @@ function stepBike(state, b, input) {
         b.state = STATE.RIDING;
         b.stateMs = 0;
         b.temp = 0;
+        emit(state, { type: 'cooldown', bike: b.index, isPlayer: b.isPlayer });
       }
       return;
     }
@@ -462,6 +467,7 @@ function stepBike(state, b, input) {
   else if (input.a) target = TUNE.speedAccel;
 
   if (surf === SURFACE.MUD && grounded(b)) target = Math.min(target, TUNE.mudSpeedCap);
+  else if (surf === SURFACE.COOL && grounded(b) && turbo) target = TUNE.coolZoneBoostSpeed;
 
   if (b.vx < target) {
     b.vx = Math.min(target, b.vx + TUNE.accelRate * DT);

tools/verifyExcitebike.js

@@ -422,7 +422,7 @@ section('4. Physics invariants');
     for (const b of state.bikes) {
       seenStates.add(b.state);
       if (![b.x, b.vx, b.y, b.vy, b.lane, b.pitch, b.temp].every(Number.isFinite)) nan += 1;
-      if (b.vx > TUNE.speedTurbo + 1 || b.vx < -0.001) overSpeed += 1;
+      if (b.vx > TUNE.coolZoneBoostSpeed + 1 || b.vx < -0.001) overSpeed += 1;
       if (b.temp < -0.001 || b.temp > 1.001) badTemp += 1;
       if (b.lane < -0.001 || b.lane > LANE_COUNT - 1 + 0.001) badLane += 1;
       if (b.state === STATE.RIDING && b.y < -0.5) sunk += 1;
@@ -431,7 +431,7 @@ section('4. Physics invariants');
   }
 
   check('nothing ever goes NaN', nan === 0, `${nan} samples`);
-  check('speed never exceeds the turbo cap', overSpeed === 0, `${overSpeed} samples`);
+  check('speed never exceeds the cool-zone boost cap', overSpeed === 0, `${overSpeed} samples`);
   check('temperature stays in 0..1', badTemp === 0, `${badTemp} samples`);
   check('bikes stay on the four lanes', badLane === 0, `${badLane} samples`);
   check('a riding bike never sinks below the ground', sunk === 0, `${sunk} samples`);