feat: add BodyTypeManual and body flags to physics2d rigidbody

[AnthonyWadham]

TL;DR

Added a new BodyTypeManual rigidbody type for gameplay-driven entities that use Box2D only for collision detection while maintaining full ECS control over position and velocity.

What changed?

• Added BodyTypeManual constant to the rigidbody component with detailed documentation explaining its behavior
• Extended Rigidbody2D struct with new body flags: Active, Awake, SleepingAllowed, Bullet, and FixedRotation
• Added NewRigidbody2D constructor function that sets Box2D-compatible defaults
• Implemented custom UnmarshalJSON method to handle backward compatibility with old snapshots while applying proper defaults
• Modified body creation to set zero velocity in Box2D for manual bodies since ECS owns their velocity
• Updated reconciliation logic to always enforce zero velocity in Box2D for manual bodies and apply all new body flags
• Created new writeback system (WritebackPhysicsSystem) that runs on PostUpdate hook to sync Box2D simulation results back to ECS components
• Added writeback logic that skips static and manual bodies, only updating dynamic and kinematic body positions/velocities from Box2D
• Updated validation to accept the new BodyTypeManual type
• Added Body function to expose underlying Box2D body for read-only queries
• Exported the new body type constant in the plugin interface

How to test?

Added up stack into the integration tests

Why make this change?

This enables better separation of concerns for gameplay-driven entities like player characters, NPCs, and AI-controlled objects. These entities can use their own movement logic (input handling, pathfinding, AI) while still benefiting from Box2D's collision detection capabilities. The manual body type prevents Box2D from overriding gameplay-computed positions while maintaining collision interactions with physics-simulated objects. The additional body flags provide fine-grained control over Box2D simulation behavior, matching the full feature set of the underlying physics engine.

[AnthonyWadham]

Warning

This pull request is not mergeable via GitHub because a downstack PR is open. Once all requirements are satisfied, merge this PR as a stack on Graphite.
Learn more

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

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