feat: add Box2D physics2d plugin with components, systems, and queries

[AnthonyWadham]

Add 2D Physics Plugin with Box2D Integration

This pull request introduces a comprehensive 2D physics plugin for Cardinal using Box2D as the simulation backend.

Components

• Transform2D: World-space position and rotation
• Velocity2D: Linear and angular velocity
• Rigidbody2D: Body type (static/dynamic/kinematic) and simulation parameters
• Collider2D: Compound collider with multiple shapes (circle, box, polygon, chain)
• ActiveContacts: Singleton component tracking active contact pairs for event deduplication

Core Systems

• InitPhysicsSystem: Creates Box2D world and performs initial ECS→Box2D rebuild
• ReconcilePhysicsSystem: Incrementally syncs ECS changes to Box2D using shadow-copy diffing
• PhysicsStepSystem: Steps the simulation and emits contact/trigger events

Contact Events

• ContactBeginEvent/ContactEndEvent: Non-sensor collision events
• TriggerBeginEvent/TriggerEndEvent: Sensor overlap events
• Proper event deduplication after world rebuilds using persisted contact state

Query API

• Raycast: Cast rays through the physics world
• OverlapAABB: Find fixtures overlapping an axis-aligned bounding box
• CircleSweep: Sweep a circle along a path for continuous collision detection

Runtime Management

• Shadow-copy reconciliation for efficient incremental updates
• Structural vs mutable change detection (fixture recreation vs in-place updates)
• Deterministic rebuild ordering and contact event handling
• Proper cleanup of orphaned bodies and contact pairs

The plugin integrates seamlessly with Cardinal's ECS architecture while providing a complete 2D physics simulation with collision detection, rigid body dynamics, and comprehensive query capabilities.

[AnthonyWadham]

• feat: physics2d Cardinal integration tests #881
• feat: add ShapeTypeStaticChainLoop and ShapeTypeEdge collider shapes #883
• feat: add BodyTypeManual and body flags to physics2d rigidbody #882
• feat: add Box2D physics2d plugin with components, systems, and queries #880 👈 (View in Graphite)
• main

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