[Feature] 2D collision shapes

[vmarcella]

Overview

Add collision shapes that can be attached to rigid bodies to enable collision
detection. Support common 2D primitives.

Current State

No collision shapes exist & requires rigid bodies to be implemented.

Scope

Goals:

• Circle collider (position, radius)
• Rectangle/box collider (half-extents)
• Capsule collider (height, radius)
• Convex polygon collider
• Attach multiple shapes to one body

Non-Goals:

• Concave/decomposed shapes (deferred)
• Collision detection callbacks
• Collision layers/masks

Proposed API

pub struct Collider2D {
  // handle
}

pub enum ColliderShape2D {
  Circle { radius: f32 },
  Rectangle { half_width: f32, half_height: f32 },
  Capsule { half_height: f32, radius: f32 },
  ConvexPolygon { vertices: Vec<[f32; 2]> },
}

pub struct Collider2DBuilder {
  shape: ColliderShape2D,
  offset: [f32; 2],
  density: f32,
  friction: f32,
  restitution: f32,
}

impl Collider2DBuilder {
  pub fn circle(radius: f32) -> Self;
  pub fn rectangle(half_width: f32, half_height: f32) -> Self;
  pub fn capsule(half_height: f32, radius: f32) -> Self;
  pub fn polygon(vertices: Vec<[f32; 2]>) -> Self;
  pub fn with_offset(self, x: f32, y: f32) -> Self;
  pub fn with_density(self, density: f32) -> Self;
  pub fn with_friction(self, friction: f32) -> Self;
  pub fn with_restitution(self, bounce: f32) -> Self;
  pub fn build(self, body: &mut RigidBody2D) -> Collider2D;
}

Acceptance Criteria

• Circle colliders detect collisions correctly
• Rectangle colliders detect collisions correctly
• Capsule colliders work for character shapes
• Polygon colliders support arbitrary convex shapes
• Multiple colliders on one body work together
• Friction and restitution affect collision response

Affected Crates

lambda-rs, lambda-rs-platform

Notes

• Density affects mass calculation
• Restitution: 0 = no bounce, 1 = perfect bounce
• Friction: 0 = ice, 1 = high grip