@fourjs/physics-rapier

Rapier solver adapters (2D and 3D, WebAssembly). Part of fourJS.

Implements §37 and §102 of docs/SPECIFICATION.md; the first adapter per Phase 5 (§108). This package sits below @fourjs/physics: it implements PhysicsSolverAdapter and depends on nothing else in the engine, which is what makes the solver swappable. Application code should target @fourjs/physics and hand it an adapter instance.

What's here

• Rapier2dAdapter and Rapier3dAdapter — one per §21 dimension, each wrapping its own @dimforge/rapier{2,3}d-compat build (pinned 0.20.0, base64-inlined wasm). Neither loads the other's module; importing the barrel costs nothing until initializeRapier2d / initializeRapier3d runs (async, §37 permits a Promise).
• RapierBodyAccess — the per-handle SolverBodyAccess implementation, declared once and dimension-independent. Both adapters also implement SolverBodyTuningAccess (§37 property changes after createBody: the §23 mass triple, damping, gravity scale, the §31 mode, and a collider's §25 material / §24 filter), so a PhysicsWorld on either of them reports supportsLiveProperties: true.
• Conversion helpers — toRapierVector2/3, fromRapierVector2/3, angle/rotation/quaternion conversions, shape and collider-descriptor builders, interaction-group packing, and principal-inertia conversion.

Notes and measured deviations (recorded in the stable API docs)

• Adapters own monotonic, never-reused body ids (Rapier handles are unordered doubles) — the §33 checksum order; snapshot envelopes F4R2/F4R3 carry the id registry.
• Rapier 0.20.0 exposes no joint reaction getters → reportsJointReactions is false and breakable joints are refused; motor maxTorque/maxForce acts as a force-based gain, not §28's hard cap.
• collisionstay is adapter-derived from a touching-pair map (Rapier reports only start/stop); restitution combine is forced to Max per Appendix A; §32 sleep thresholds are unmapped (only enabled).

Unit tests are colocated in tests/ per §92.

Workspace name @fourjs/physics-rapier; publishes as @danielsimonjr/fourjs-physics-rapier.