Fix camera judder on fast ascent

Root cause: startFollow used lerpY=0.05 + roundPixels=true. At rocket
velocity (~1400 px/s) the camera lagged ~466px behind the player, then
snapped in chunks every frame, giving the impression of low FPS / shake.

Fix:
- Replace startFollow with manual upward-only camera tracking that
  matches the player exactly (doodle-jump standard behavior)
- Disable camera roundPixels so antialiasing actually smooths motion
- Round bg tilePositionY to integer to avoid grid texture shimmer
- Slightly thin out boost effects (flame 3->2/frame, speed line 40->65ms)
  to keep frame budget headroom on weaker devices

src/managers/EffectsManager.js

@@ -21,7 +21,7 @@ export class EffectsManager {
       else this._spawnWind(player);
 
       this.speedLineTimer += delta;
-      if (this.speedLineTimer > 40) {
+      if (this.speedLineTimer > 65) {
         this.speedLineTimer = 0;
         this._spawnSpeedLine();
       }
@@ -70,7 +70,7 @@ export class EffectsManager {
 
   _spawnFlame(player) {
     const baseY = player.y + player.displayHeight / 2 - 6;
-    const count = 3;
+    const count = 2;
     for (let i = 0; i < count; i++) {
       const jitter = Phaser.Math.Between(-9, 9);
       const color = Phaser.Utils.Array.GetRandom(FLAME_COLORS);

src/scenes/GameScene.js

@@ -60,7 +60,10 @@ export class GameScene extends Scene {
     this.physics.add.overlap(this.player, this.platformManager.getEnemies(), this.handleEnemy, null, this);
 
     this.cameras.main.setBounds(0, -999999, GAME_WIDTH, 999999 + GAME_HEIGHT);
-    this.cameras.main.startFollow(this.player, true, 0, 0.05, 0, 180);
+    // Manual upward-only follow: camera rises with player but never drops.
+    // Avoids lerp lag at high boost velocities that looked like FPS judder.
+    this.cameraOffsetY = 180;
+    this.cameras.main.setRoundPixels(false);
 
     this.isGameOver = false;
     this.isPaused = false;
@@ -83,7 +86,13 @@ export class GameScene extends Scene {
 
     this.player.update(this.cursors, this.wasd, this.touchLeft, this.touchRight, time, delta);
 
-    this.bg.tilePositionY = this.cameras.main.scrollY * 0.3;
+    // Camera follow: instantly track player on the way up, never drop.
+    const targetY = this.player.y - GAME_HEIGHT / 2 + this.cameraOffsetY;
+    if (targetY < this.cameras.main.scrollY) {
+      this.cameras.main.scrollY = targetY;
+    }
+
+    this.bg.tilePositionY = Math.round(this.cameras.main.scrollY * 0.3);
 
     this.minScrollY = Math.min(this.minScrollY, this.cameras.main.scrollY);
     const killLine = this.minScrollY + GAME_HEIGHT;