Trigger object SFX on physical impact with force-scaled volume

[dalision]

Problem

Object impact SFX in worlds/<slug>/output/<object>/sfx/ currently only play once, at the moment of click-to-grab (useObjectGrab.ts → playInteractionSfx). Actual physics collisions — object hitting the floor after a throw, smacking into a wall, knocking into another object — fire no sound at all.

This makes the impact samples (recorded by the SFX pipeline specifically to sound like physical contact) feel disconnected from the physics: you can chuck a chair across the room, see it ragdoll, hear nothing on landing.

Change

SceneObject.tsx wires Rapier's onContactForce on each dynamic RigidBody. Three guards keep the audio usable:

1. Force threshold (IMPACT_FORCE_THRESHOLD = 8) — skips below-threshold contacts so an object resting on the ground stays quiet.
2. Throttle (IMPACT_THROTTLE_MS = 200) — at most one playback per object per 200ms, so a bouncing/sliding contact stream doesn't machine-gun the same sample.
3. Force-proportional volume — volume = totalForceMagnitude / IMPACT_FORCE_FULL_VOLUME, clamped to [0.15, 1.0] so light taps stay audible while hard impacts hit max.

playRandomSfx gains an optional volume argument (defaults to 1) so the existing click-to-grab path is unchanged.

Net diff: +17 / -2 in one file, no new dependencies.

Verification

• bun --cwd=app run typecheck passes
• Manually validated on a fresh `cozy-room` world with two rigidbody objects (wooden chair, potted plant): click-to-grab still plays a sample (no behaviour change), and tossing them into the floor / walls / each other now triggers an appropriately scaled impact sound from the per-object sample pool. Light landings sound soft, hard throws hit max volume, resting-on-the-ground silence is preserved.

Notes

• The three constants are conservative defaults — could move to `useDebugStore` if you want per-session tuning via Leva.
• `IMPACT_FORCE_FULL_VOLUME = 80` was calibrated by feel against Rapier's default solver settings; might need re-tuning if anyone changes `linearDamping` / `additionalSolverIterations`.
• Intentionally does not differentiate impact target (floor vs wall vs another object) — would need a material lookup table for that.