refactor(lambda-rs-platform): split rapier2d implementation across multiple (#202)

## Summary

Refactor the internal `lambda-rs-platform` 2D Rapier backend from a
single
large source file into a `rapier2d` module. This keeps the
existing backend behavior and public exports unchanged while making the
code
easier to navigate and maintain.

## Related Issues

N/A

## Changes

- Replace `crates/lambda-rs-platform/src/physics/rapier2d.rs` with a
folder-backed module at
`crates/lambda-rs-platform/src/physics/rapier2d/`.
- Keep the existing `physics::rapier2d` module path and
`PhysicsBackend2D`
  exports stable through `rapier2d/mod.rs`.
- Split the backend implementation by responsibility:
  - `rigid_bodies.rs`
  - `colliders.rs`
  - `queries.rs`
  - `simulation.rs`
  - `helpers.rs`
  - `tests.rs`
- Move the existing unit tests into `tests.rs` without changing their
  assertions or backend semantics.

## Type of Change

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

## Affected Crates

- [ ] `lambda-rs`
- [x] `lambda-rs-platform`
- [ ] `lambda-rs-args`
- [ ] `lambda-rs-logging`
- [ ] Other:

## Checklist

- [x] 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
- [ ] Public API changes are documented
- [ ] Breaking changes are noted in this PR description

## Testing

**Commands run:**

```bash
cargo +nightly fmt --all
cargo test -p lambda-rs-platform rapier2d -- --nocapture
cargo test -p lambda-rs-platform --features physics-2d --lib physics::rapier2d -- --nocapture
```

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

## Screenshots/Recordings

N/A

## Platform Testing

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

## Additional Notes

Author: vmarcella

crates/lambda-rs-platform/src/physics/rapier2d.rs

@@ -0,0 +1,279 @@
+//! Rapier-backed 2D physics backend.
+//!
+//! This module provides a minimal wrapper around `rapier2d` to support the
+//! higher-level `lambda-rs` physics APIs without exposing vendor types outside
+//! of the platform layer.
+
+use std::{
+  collections::{
+    HashMap,
+    HashSet,
+  },
+  error::Error,
+  fmt,
+};
+
+use rapier2d::prelude::*;
+
+mod colliders;
+mod helpers;
+mod queries;
+mod rigid_bodies;
+mod simulation;
+
+#[cfg(test)]
+mod tests;
+
+/// The rigid body integration mode.
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub enum RigidBodyType2D {
+  /// A body that does not move under simulation.
+  Static,
+  /// A body affected by gravity and forces.
+  Dynamic,
+  /// A body integrated only by user-provided motion.
+  Kinematic,
+}
+
+/// Backend errors for 2D rigid body operations.
+#[derive(Debug, Clone, Copy, PartialEq)]
+pub enum RigidBody2DBackendError {
+  /// The referenced rigid body was not found.
+  BodyNotFound,
+  /// The provided position is invalid.
+  InvalidPosition { x: f32, y: f32 },
+  /// The provided rotation is invalid.
+  InvalidRotation { radians: f32 },
+  /// The provided linear velocity is invalid.
+  InvalidVelocity { x: f32, y: f32 },
+  /// The provided force is invalid.
+  InvalidForce { x: f32, y: f32 },
+  /// The provided impulse is invalid.
+  InvalidImpulse { x: f32, y: f32 },
+  /// The provided dynamic mass is invalid.
+  InvalidMassKg { mass_kg: f32 },
+  /// The requested operation is unsupported for the body type.
+  UnsupportedOperation { body_type: RigidBodyType2D },
+}
+
+impl fmt::Display for RigidBody2DBackendError {
+  fn fmt(&self, formatter: &mut fmt::Formatter<'_>) -> fmt::Result {
+    match self {
+      Self::BodyNotFound => {
+        return write!(formatter, "rigid body not found");
+      }
+      Self::InvalidPosition { x, y } => {
+        return write!(formatter, "invalid position: ({x}, {y})");
+      }
+      Self::InvalidRotation { radians } => {
+        return write!(formatter, "invalid rotation: {radians}");
+      }
+      Self::InvalidVelocity { x, y } => {
+        return write!(formatter, "invalid velocity: ({x}, {y})");
+      }
+      Self::InvalidForce { x, y } => {
+        return write!(formatter, "invalid force: ({x}, {y})");
+      }
+      Self::InvalidImpulse { x, y } => {
+        return write!(formatter, "invalid impulse: ({x}, {y})");
+      }
+      Self::InvalidMassKg { mass_kg } => {
+        return write!(formatter, "invalid mass_kg: {mass_kg}");
+      }
+      Self::UnsupportedOperation { body_type } => {
+        return write!(
+          formatter,
+          "unsupported operation for body_type: {body_type:?}"
+        );
+      }
+    }
+  }
+}
+
+impl Error for RigidBody2DBackendError {}
+
+/// Backend errors for 2D collider operations.
+#[derive(Debug, Clone, Copy, PartialEq)]
+pub enum Collider2DBackendError {
+  /// The referenced rigid body was not found.
+  BodyNotFound,
+  /// The provided polygon could not be represented as a convex hull.
+  InvalidPolygonDegenerate,
+}
+
+impl fmt::Display for Collider2DBackendError {
+  fn fmt(&self, formatter: &mut fmt::Formatter<'_>) -> fmt::Result {
+    match self {
+      Self::BodyNotFound => {
+        return write!(formatter, "rigid body not found");
+      }
+      Self::InvalidPolygonDegenerate => {
+        return write!(formatter, "invalid polygon: degenerate");
+      }
+    }
+  }
+}
+
+impl Error for Collider2DBackendError {}
+
+/// Backend-agnostic data describing the nearest 2D raycast hit.
+#[derive(Debug, Clone, Copy, PartialEq)]
+pub struct RaycastHit2DBackend {
+  /// The hit rigid body's slot index.
+  pub body_slot_index: u32,
+  /// The hit rigid body's slot generation.
+  pub body_slot_generation: u32,
+  /// The world-space hit point.
+  pub point: [f32; 2],
+  /// The world-space unit hit normal.
+  pub normal: [f32; 2],
+  /// The non-negative hit distance in meters.
+  pub distance: f32,
+}
+
+/// Indicates whether a backend collision pair started or ended contact.
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub enum CollisionEventKind2DBackend {
+  /// The body pair started touching during the current backend step.
+  Started,
+  /// The body pair stopped touching during the current backend step.
+  Ended,
+}
+
+/// Backend-agnostic data describing one 2D collision event.
+#[derive(Debug, Clone, Copy, PartialEq)]
+pub struct CollisionEvent2DBackend {
+  /// The transition kind for the body pair.
+  pub kind: CollisionEventKind2DBackend,
+  /// The first rigid body's slot index.
+  pub body_a_slot_index: u32,
+  /// The first rigid body's slot generation.
+  pub body_a_slot_generation: u32,
+  /// The second rigid body's slot index.
+  pub body_b_slot_index: u32,
+  /// The second rigid body's slot generation.
+  pub body_b_slot_generation: u32,
+  /// The representative world-space contact point, when available.
+  pub contact_point: Option<[f32; 2]>,
+  /// The representative world-space contact normal, when available.
+  pub normal: Option<[f32; 2]>,
+  /// The representative penetration depth, when available.
+  pub penetration: Option<f32>,
+}
+
+/// The fallback mass applied to dynamic bodies before density colliders exist.
+const DYNAMIC_BODY_FALLBACK_MASS_KG: f32 = 1.0;
+
+/// Stores per-body state that `lambda-rs` tracks alongside Rapier.
+///
+/// This slot exists because `lambda-rs` defines integration semantics that are
+/// stricter than the vendor backend:
+/// - Forces are accumulated and cleared explicitly by the public API.
+/// - Impulses update velocity immediately.
+///
+/// # Invariants
+/// - `rapier_handle` MUST reference a body in `PhysicsBackend2D::bodies`.
+/// - `explicit_dynamic_mass_kg` MUST be `Some` only for dynamic bodies.
+/// - `generation` MUST be non-zero and is used to validate handles.
+#[derive(Debug, Clone, Copy)]
+struct RigidBodySlot2D {
+  /// The rigid body's integration mode.
+  body_type: RigidBodyType2D,
+  /// The handle to the Rapier rigid body stored in the `RigidBodySet`.
+  rapier_handle: RigidBodyHandle,
+  /// Accumulated forces applied by the public API, in Newtons.
+  force_accumulator: [f32; 2],
+  /// The explicitly configured body mass in kilograms, if set.
+  ///
+  /// When this value is `Some`, collider density MUST NOT affect body mass
+  /// properties. The backend enforces this by creating attached colliders with
+  /// zero density and using the configured value as the body's additional mass.
+  explicit_dynamic_mass_kg: Option<f32>,
+  /// Tracks whether the body has at least one positive-density collider.
+  ///
+  /// This flag supports the spec requirement that bodies with no positive
+  /// density colliders default to `1.0` kg, while bodies with at least one
+  /// positive-density collider compute mass from collider density alone.
+  has_positive_density_colliders: bool,
+  /// A monotonically increasing counter used to validate stale handles.
+  generation: u32,
+}
+
+/// Stores per-collider state that `lambda-rs` tracks alongside Rapier.
+///
+/// # Invariants
+/// - `rapier_handle` MUST reference a collider in `PhysicsBackend2D::colliders`.
+/// - `generation` MUST be non-zero and is used to validate stale handles.
+#[derive(Debug, Clone, Copy)]
+struct ColliderSlot2D {
+  /// The handle to the Rapier collider stored in the `ColliderSet`.
+  rapier_handle: ColliderHandle,
+  /// The parent rigid body slot index that owns this collider.
+  parent_slot_index: u32,
+  /// The parent rigid body slot generation that owns this collider.
+  parent_slot_generation: u32,
+  /// A monotonically increasing counter used to validate stale handles.
+  generation: u32,
+}
+
+/// Describes how collider attachment should affect dynamic-body mass semantics.
+///
+/// This helper isolates `lambda-rs` mass rules from the Rapier attachment flow
+/// so body creation and collider attachment share one backend policy source.
+#[derive(Debug, Clone, Copy, PartialEq)]
+struct ColliderAttachmentMassPlan2D {
+  /// The density value that MUST be passed to the Rapier collider builder.
+  rapier_density: f32,
+  /// Whether attaching this collider transitions the body to density-driven
+  /// mass computation.
+  should_mark_has_positive_density_colliders: bool,
+  /// Whether the initial fallback mass MUST be removed before insertion.
+  should_remove_fallback_mass: bool,
+}
+
+/// A normalized body-pair key used for backend collision tracking.
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
+struct BodyPairKey2D {
+  /// The first body slot index.
+  body_a_slot_index: u32,
+  /// The first body slot generation.
+  body_a_slot_generation: u32,
+  /// The second body slot index.
+  body_b_slot_index: u32,
+  /// The second body slot generation.
+  body_b_slot_generation: u32,
+}
+
+/// The representative contact selected for a body pair during one step.
+#[derive(Debug, Clone, Copy, PartialEq)]
+struct BodyPairContact2D {
+  /// The representative world-space contact point.
+  point: [f32; 2],
+  /// The representative world-space normal from body A toward body B.
+  normal: [f32; 2],
+  /// The non-negative penetration depth.
+  penetration: f32,
+}
+
+/// A 2D physics backend powered by `rapier2d`.
+///
+/// This type is an internal implementation detail used by `lambda-rs`.
+pub struct PhysicsBackend2D {
+  gravity: Vector,
+  integration_parameters: IntegrationParameters,
+  islands: IslandManager,
+  broad_phase: BroadPhaseBvh,
+  narrow_phase: NarrowPhase,
+  bodies: RigidBodySet,
+  colliders: ColliderSet,
+  impulse_joints: ImpulseJointSet,
+  multibody_joints: MultibodyJointSet,
+  ccd_solver: CCDSolver,
+  pipeline: PhysicsPipeline,
+  rigid_body_slots_2d: Vec<RigidBodySlot2D>,
+  collider_slots_2d: Vec<ColliderSlot2D>,
+  collider_parent_slots_2d: HashMap<ColliderHandle, (u32, u32)>,
+  active_body_pairs_2d: HashSet<BodyPairKey2D>,
+  active_body_pair_order_2d: Vec<BodyPairKey2D>,
+  queued_collision_events_2d: Vec<CollisionEvent2DBackend>,
+}