Box2D solver adapter — a reserved stub. This package contains no implementation today. Part of fourJS.
What a consumer gets by installing it: one export, PACKAGE_NAME, and nothing else — measured from a consumer seat on 2026-09-19 (dogfood cycle 9) against installed tarballs, in Node and in Chrome. There is no register* function and no adapter class. fourJS/physics-box2d imports cleanly and registers no solver, so selecting solver: "box2d" fails with the registry's "no physics solver is registered" message (§37), not with a module error. The shipped solver capability lives in @fourjs/physics-rapier (Rapier2dAdapter, Rapier3dAdapter); unlike the renderer seam, there is no consumer-side workaround short of writing a whole §37 adapter — see docs/guides/custom-solver-adapters.md for that contract.
Reserved for the 2D solver adapter backed by Box2D, implementing @fourjs/physics's PhysicsSolverAdapter (§37) plus the SolverBodyAccess/SolverJointAccess seams, and declaring its capability differences per §102 in docs/SPECIFICATION.md.
The package exists in the workspace so the §98 monorepo tree and the §102 solver list stay accurate (see ERRATA E-3: physics-rapier, physics-box2d, and physics-soft are the deliberate set — no physics-matter or physics-cannon without a spec amendment). The barrel currently exports only PACKAGE_NAME, and tests/ holds a single smoke test.
One recorded motivation for this adapter: Box2D could honor §28's motor maxTorque/maxForce as a real hard cap, which Rapier treats as a force-based gain (capability-table item).
Unit tests are colocated in tests/ per §92.
Workspace name @fourjs/physics-box2d; publishes as @danielsimonjr/fourjs-physics-box2d.