feat(lambda-rs): Add 2d collision events (#201)

## Summary

Add 2D collision queries, collision filtering, and collision start/end
events
to the `physics-2d` feature.

## Related Issues

- Resolves #121 


## Changes

- Add public 2D physics API scaffolding in `lambda-rs`:
  - `CollisionEventKind`
  - `CollisionEvent`
  - `CollisionFilter`
  - `RaycastHit`
  - `PhysicsWorld2D::collision_events()`
  - `PhysicsWorld2D::query_point()`
  - `PhysicsWorld2D::query_aabb()`
  - `PhysicsWorld2D::raycast()`
  - `Collider2DBuilder::with_collision_filter()`
- Add backend support in `lambda-rs-platform` Rapier integration for:
  - collider collision group/mask filtering
  - point queries
  - AABB overlap queries
  - nearest-hit raycasts
  - body-pair `Started` collision events
  - body-pair `Ended` collision events
- compound-body collider aggregation into one public body-pair event
stream
- Add public-to-backend handle mapping support so query and event
results can
  be reconstructed as stable `RigidBody2D` handles
- Add focused integration tests for:
  - collision filters
  - point, AABB, and ray queries
  - collision start events
  - collision end events
  - queue draining behavior
  - compound-body deduplication behavior
- Add a new demo binary:
  - `demos/physics/src/bin/physics_collision_events_2d.rs`
  - shows floor/ball contact start and end events
  - logs event data and changes ball tint while contact is active
- Add and update documentation:
  - new spec for 2D collision queries and events
  - spec index update
  - `docs/features.md` update for `physics-2d`
  - new tutorial for the collision events demo
  - tutorial index update

## Type of Change

- [ ] Bug fix (non-breaking change that fixes an issue)
- [x] Feature (non-breaking change that adds functionality)
- [ ] Breaking change (fix or feature that would cause existing
functionality to change)
- [x] Documentation (updates to docs, specs, tutorials, or comments)
- [ ] Refactor (code change that neither fixes a bug nor adds a feature)
- [ ] Performance (change that improves performance)
- [x] Test (adding or updating tests)
- [ ] Build/CI (changes to build process or CI configuration)

## Affected Crates

- [x] `lambda-rs`
- [x] `lambda-rs-platform`
- [ ] `lambda-rs-args`
- [ ] `lambda-rs-logging`
- [x] Other:
  `demos/physics`, `docs/specs`, `docs/tutorials`

## Checklist

- [x] Code follows the repository style guidelines (`cargo +nightly fmt
--all`)
- [x] Code passes clippy (`cargo clippy --workspace --all-targets -- -D
warnings`)
- [x] Tests pass (`cargo test --workspace`)
- [x] New code includes appropriate documentation
- [x] Public API changes are documented
- [ ] Breaking changes are noted in this PR description

## Testing

**Commands run:**

```bash
cargo +nightly fmt --all
cargo build -p lambda-rs --features physics-2d
cargo test -p lambda-rs --features physics-2d -- --nocapture
cargo test -p lambda-rs --features physics-2d collision_filters -- --nocapture
cargo test -p lambda-rs --features physics-2d queries -- --nocapture
cargo test -p lambda-rs --features physics-2d collision_events -- --nocapture
cargo build -p lambda-demos-physics --bin physics_collision_events_2d
cargo clippy --workspace --all-targets -- -D warnings
cargo test --workspace
```

**Manual verification steps (if applicable):**

1. Run `cargo run -p lambda-demos-physics --bin
physics_collision_events_2d`.
2. Observe the ball falling onto the floor and a `Started` event being
logged.
3. Press `Space` while the ball is grounded to apply the launch impulse.
4. Observe the ball leaving the floor, a corresponding `Ended` event,
and the
   ball tint returning to its non-contact state.

## Screenshots/Recordings

Not included. The main visual change is the new
`physics_collision_events_2d` demo.

## Platform Testing

- [x] macOS
- [ ] Windows
- [ ] Linux

## Additional Notes

- Queries intentionally scan the live collider set so they work before
the
  first simulation step.
- `collision_events()` is a post-step drain rather than a callback
system,
  which keeps gameplay logic outside backend simulation execution.

Author: vmarcella

crates/lambda-rs-platform/src/physics/mod.rs

@@ -8,7 +8,10 @@ pub mod rapier2d;
 
 pub use rapier2d::{
   Collider2DBackendError,
+  CollisionEvent2DBackend,
+  CollisionEventKind2DBackend,
   PhysicsBackend2D,
+  RaycastHit2DBackend,
   RigidBody2DBackendError,
   RigidBodyType2D,
 };