Physics: add continuous collision detection (CCD) for fast-moving bodies

[stormmuller]

Problem

src/physics only performs discrete collision detection: a body's position is integrated each tick and then checked for overlap. A fast-moving body can fully pass through a thin collider (e.g. a TerrainCollider wall, a thin PolygonCollider) within a single tick without ever registering an overlapping AABB — classic tunneling.

Impact

Any gameplay involving high-speed projectiles, fast-moving characters, or thin walls/floors is at risk of objects passing straight through geometry they should collide with.

Suggested fix

Add continuous collision detection for bodies moving fast relative to their own size and/or nearby collider thickness — options include swept-shape (conservative advancement) checks between the previous and current tick's transform, or sub-stepping the integration for qualifying bodies. Should be opt-in/threshold-based so it doesn't add cost to the common slow-moving-body case (consistent with how Box2D gates CCD on a speed threshold).

Acceptance criteria

• A fast-moving circle/polygon body reliably collides with thin static geometry instead of tunneling through it
• CCD only engages for bodies/situations that need it, to avoid broad performance regression
• Covered by a physics unit test that reproduces tunneling without the fix and passes with it
• Documented in documentation-site/docs/docs/physics