[Feature] Drive external physics backends via an EngineExtension coordinator

[miogds]

Summary

PR B of the narrowed phase-1 plan from discussion #1116 (follows #1123), reworked per your review guidance to sit entirely on the #1127 EngineExtension infrastructure — no runFrame seam, no engine-core changes. Rebased on current develop (post the repo-wide format commit); make lint passes clean on the branch. With no backend installed, nothing is even scheduled: the coordinator is only registered while a backend plugin is installed, so runtime behavior is unchanged by construction. The built-in integrator is untouched and keeps running legacy PhysicsComponents/KineticComponent entities in all cases (the coexistence rule from the plan).

• Sources/UntoldEngine/Physics/PhysicsCoordinator.swift — PhysicsCoordinator conforming to EngineExtension (id: com.untoldengine.physics.coordinator). Its fixedUpdate runs once per fixed substep right after updatePhysicsSystem, and per substep: (1) diffs the RigidBodyComponent+ColliderComponent+LocalTransformComponent query result against the backend's known-body set → didAddBody/didRemoveBody (entity destruction needs no extra hook — component cleanup drops the entity out of the query and the diff catches it); (2) batch-writes kinematic targets; (3) backend.step; (4) batch-reads active transforms back into LocalTransformComponent for dynamic bodies, marking transformDirty/anyTransformDirty exactly like the built-in integrator; (5) drains events into an inert sink (delivery/USC wiring is PR C).
• PhysicsBackendRegistry now registers the coordinator with EngineExtensionRegistry.shared.register(_:) on successful install (also pushing the current PhysicsWorldConfiguration to the backend) and unregisters it on uninstall — both outside the registry lock. The lockForRuntime() seam from [Feature] Add physics backend plugin interface #1123 is wired with zero core edits: the coordinator locks the registry on the first substep it simulates, freezing the backend choice for the run.
• Configurable gravity: PhysicsCoordinator.setWorldConfiguration(_:) applies to the active backend and to the built-in integrator, whose hardcoded (0, -9.8, 0) in updatePhysicsSystem now reads the same configuration (default identical, hoisted out of the per-entity loop).
• 15 unit tests with a recording mock backend: install/uninstall scheduling, world-config propagation, body add/remove lifecycle (destroy and component removal), descriptor snapshot fidelity, kinematic batch writes, dynamic read-back (including ignoring kinematic/unknown entities), per-substep call order, runtime lock, and built-in-integrator gravity parity/configurability.

Engine touch outside Physics/: one hoisted gravity read in Systems/PhysicsSystem.swift (3 lines). Follow-ups per the plan: PR C (event sink subscriptions + USC OnCollision), PR D (raycast facade).

Test plan

• swift test --filter PhysicsCoordinatorTests — 15/15 pass
• PhysicsBackendRegistryTests, ComponentRegistryTest, ECSTests, EngineExtensionLifecycleTest, CustomSystemTest — all pass
• Full UntoldEngineTests module: 1060 tests, no new failures (the one failure, ExternalRenderExtensionPackageTests, fails identically on clean develop on this machine)