feat(lambda-rs): add 2D colliders (#195)

## Summary

Add 2D collider support on top of the rigid body implementation,
including
public builder APIs in `lambda-rs`, Rapier-backed shape attachment in
`lambda-rs-platform`, integration coverage, and a demo that exercises
material,
density, local transform, and compound-collider behavior.

This PR introduces circle, rectangle, capsule, and convex polygon
colliders,
supports attaching multiple colliders to a single rigid body, and
verifies
collision response through integration tests and a visual demo.

## Related Issues

N/A

## Changes

- Add `Collider2D`, `ColliderShape2D`, `Collider2DBuilder`, and collider
  validation/error handling to `lambda-rs`
- Add Rapier-backed circle, rectangle, capsule, and convex polygon
collider
  attachment to `lambda-rs-platform`
- Support compound colliders by allowing multiple collider attachments
per
  rigid body
- Add density, friction, restitution, local offset, and local rotation
support
  to collider construction
- Add physics integration test modules for collider shapes, materials,
and
  compound-collider behavior
- Add a dedicated `physics_colliders_2d` demo covering primitives,
materials,
  density, local rotation, and compound colliders
- Centralize collider validation in `lambda-rs` and reduce backend error
  handling to backend-owned failures
- Replace indirect body validation with direct live-handle validation
- Remove the custom Rapier contact hook and encode friction/restitution
through
  Rapier combine rules
- Reduce unnecessary per-step backend work by skipping zero-force
accumulator
  synchronization

## Type of Change

- [x] 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)
- [x] Refactor (code change that neither fixes a bug nor adds a feature)
- [x] 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`

## Checklist

- [x] Code follows the repository style guidelines (`cargo +nightly fmt
--all`)
- [x] Code passes clippy (`cargo clippy --workspace --all-targets -- -D
warnings`)
- [ ] 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-platform --features physics-2d
cargo build -p lambda-rs --features physics-2d
cargo test -p lambda-rs --features physics-2d collider_2d::tests:: -- --nocapture
cargo test -p lambda-rs-platform --features physics-2d rigid_body_exists_2d_reports_live_slots -- --nocapture
cargo test -p lambda-rs-platform --features physics-2d collider_attachment_mass_plan -- --nocapture
cargo test -p lambda-rs-platform --features physics-2d rapier_friction_encoding_preserves_public_combination_semantics -- --nocapture
cargo test -p lambda-rs --features physics-2d physics_2d_friction_changes_sliding_velocity_decay -- --nocapture
cargo test -p lambda-rs --features physics-2d physics_2d_restitution_changes_bounce_height -- --nocapture
cargo test -p lambda-rs --features physics-2d physics_2d_density_affects_impulse_velocity_delta -- --nocapture
cargo clippy --workspace --all-targets --features physics-2d -- -D warnings
```

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

1. Run `cargo run -p lambda-demos-physics --bin physics_colliders_2d
--features physics-2d`
2. Verify circle, rectangle, capsule, and convex polygon bodies collide
with
   the floor, ramp, walls, and divider
3. Verify friction changes sliding distance, restitution changes bounce,
and
   density changes response to the applied impulse

## Screenshots/Recordings

Not included in this draft.

## Platform Testing

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

## Additional Notes

- Full workspace tests were not run as part of this draft. Two
force-lifetime
  tests were previously observed failing independently of the collider
  simplification work and should be evaluated separately before merge.

Author: vmarcella

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

@@ -7,6 +7,7 @@
 pub mod rapier2d;
 
 pub use rapier2d::{
+  Collider2DBackendError,
   PhysicsBackend2D,
   RigidBody2DBackendError,
   RigidBodyType2D,