[Feature] 2D collision detection and callbacks #121

Overview

Add collision detection queries and event callbacks so games can respond to
physics interactions between bodies.

Current State

No response

Scope

Goals:

• Collision start/end events
• Contact point information
• Collision filtering (layers/masks)
• Query overlapping bodies at a point
• Intersection tests without simulation

Non-Goals:

• Trigger volumes (separate issue)
• Complex filtering rules (deferred)

Proposed API

pub struct CollisionEvent {
  pub body_a: RigidBodyHandle,
  pub body_b: RigidBodyHandle,
  pub contact_point: [f32; 2],
  pub normal: [f32; 2],
  pub penetration: f32,
}

pub struct CollisionFilter {
  pub group: u32,
  pub mask: u32,
}

impl PhysicsWorld2D {
  pub fn collision_events(&self) -> impl Iterator<Item = CollisionEvent>;
  pub fn query_point(&self, point: [f32; 2]) -> Vec<RigidBodyHandle>;
  pub fn query_aabb(&self, min: [f32; 2], max: [f32; 2]) -> Vec<RigidBodyHandle>;
  pub fn raycast(&self, origin: [f32; 2], dir: [f32; 2], max_dist: f32) 
    -> Option<RaycastHit>;
}

impl Collider2DBuilder {
  pub fn with_collision_filter(self, filter: CollisionFilter) -> Self;
}

Acceptance Criteria

• Collision events fire when bodies first touch
• Collision events include contact information
• Collision filters prevent specified pairs from colliding
• Point queries find bodies containing that point
• Raycasts return first hit with normal and distance

Affected Crates

lambda-rs, lambda-rs-platform

Notes

• Events collected during step, consumed after
• Filter: collide if (a.group & b.mask) && (b.group & a.mask)