feat(lambda-rs): 2D rigid body support (#185)

## Summary

Add 2D rigid body support to `PhysicsWorld2D` with a backend-
agnostic public API in `lambda-rs`, a Rapier-backed implementation in
`lambda-rs-platform`, and supporting documentation + a runnable demo.

This work adds:
- Rigid body creation for `Static`, `Dynamic`, and `Kinematic` bodies.
- Query + mutation of position, rotation, and linear velocity.
- Force + impulse application for dynamic bodies.
- A safe handle model that prevents world mismatches and stale body
access.
- A demo and tutorial showcasing usage without collision
shapes/response.

## Related Issues

- Resolves #119 

## Changes

- Add `RigidBodyType`, `RigidBody2D`, and `RigidBody2DBuilder`.
- Expose body operations as fallible APIs with actionable errors:
  - World scoping: `WorldMismatch`, `InvalidHandle`
  - Liveness: `BodyNotFound`
- Input validation: `InvalidPosition`, `InvalidRotation`,
`InvalidVelocity`,
    `InvalidForce`, `InvalidImpulse`, `InvalidMassKg`
  - Capability gating: `UnsupportedOperation { body_type }`
- Implement fixed-timestep stepping semantics:
  - The backend advances once per substep.
- Accumulated forces are cleared once per outer
`PhysicsWorld2D::step()`.
- Implement integration semantics:
- Dynamic bodies: symplectic Euler with gravity + accumulated
force/mass.
  - Kinematic bodies: position integrates from velocity.
  - Static bodies: no integration.
- Store bodies in slot storage with a generation counter to prevent
stale-handle
  aliasing when slots are reused.
- Add `lambda-demos-physics` runnable demo: `physics_rigid_bodies_2d`.
- Demonstrates dynamic bodies (gravity/forces/impulses), a kinematic
body
    (programmatic motion), and a static reference body.
- Uses manual floor/wall/ceiling constraints to keep bodies on-screen
(no
    collision shapes/response).
- Add spec: `docs/specs/physics/rigid-bodies-2d.md` and update spec
index.
- Add tutorial: `docs/tutorials/physics/basics/rigid-bodies-2d.md` and
update
  tutorial index.
- Update `docs/features.md` for the new physics-2D feature surface
without
  pinning a specific Rapier version.

## 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: `lambda-demos-physics`

## Checklist

- [ ] Code follows the repository style guidelines (`cargo +nightly fmt
--all`)
- [ ] 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 clippy -p lambda-demos-physics --all-targets --features physics-2d -- -D warnings
```

**Recommended additional verification:**

```bash
cargo build --workspace
cargo test --workspace
cargo clippy --workspace --all-targets -- -D warnings
```

**Manual verification steps:**

1. `cargo run -p lambda-demos-physics --bin physics_rigid_bodies_2d
--features physics-2d`
2. Observe:
   - Dynamic bodies fall under gravity and drift under wind force.
   - Space applies an upward impulse to the dynamic bodies.
   - Bodies remain constrained within floor/walls/ceiling bounds.
- Kinematic body motion is driven by velocity and remains within wall
bounds.

## Screenshots/Recordings

## Platform Testing

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

## Additional Notes

- Collision shapes/response are intentionally out of scope; the demo
constraints
  are explicit logic to keep bodies visible.

Author: vmarcella

.pre-commit-config.yaml

@@ -22,7 +22,7 @@ repos:
         name: Linting for lambda-rs & tools.
         types: [rust]
       - id: clippy
-        entry: rustup run nightly cargo clippy --workspace --all-targets -- -D warnings
+        entry: cargo clippy --workspace --all-targets -- -D warnings
         language: system
         name: Clippy for lambda-rs & tools.
         pass_filenames: false

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

@@ -6,4 +6,8 @@
 
 pub mod rapier2d;
 
-pub use rapier2d::PhysicsBackend2D;
+pub use rapier2d::{
+  PhysicsBackend2D,
+  RigidBody2DBackendError,
+  RigidBodyType2D,
+};