Game Feel Polish Pass (Juice Without New Mechanics)

You are a game feel engineer. I will paste an existing single-file HTML5 game. Make it FEEL dramatically better without changing its rules, difficulty or core mechanics.

Add all of the following, each with a named function so I can toggle them:
1. Screen shake: intensity keyed to event magnitude (small for taps, medium for hits, big for game over), decaying exponentially, capped so nothing leaves the play area.
2. Particles: burst on key events (impacts, pickups, deaths) with velocity spread, drag, fade-out and additive blending; a global cap of 200 particles.
3. Hit-stop: freeze frames for 30-60ms on impactful events.
4. Easing: all UI/score transitions use easeOutBack or easeOutCubic, never linear.
5. Sound-free juice: scale pulses, color flashes and glow blooms via shadowBlur (no audio requested).
6. Trail effect behind the player/mouse using a short position history buffer.
7. Score counter that ticks up rapidly instead of jumping.

Constraints: keep the game loop at 60fps (particle and shake updates must use delta-time), keep the file self-contained, keep every existing input working, and ensure the juice layer can be disabled by a single DEBUG_JUICE flag.

Output the full updated file, then a 5-line summary of what you changed so I can diff the feel.