[Feature] Add physics backend plugin interface

[miogds]

Summary

PR A of the physics plugin readiness plan (upstream discussion #1116): the plugin-facing seam for optional physics backends. Nothing invokes the new API yet, so runtime behavior is unchanged by construction.

• Sources/UntoldEngine/Physics/PhysicsBackend.swift — PhysicsBackend protocol with the threading contract (step/drain on the frame thread, backends buffer worker-thread events internally) and batch-only transform transfer, plus all value types: PhysicsCapabilities, PhysicsMotionType, PhysicsColliderShape, PhysicsWorldConfiguration (gravity default (0, -9.8, 0), collision layer matrix, m/kg/s units), body/collider descriptors, write/read transform batches, contact/trigger/activation events, PhysicsEventSink, ray query types. Default no-op implementations so minimal backends only implement what they support.
• Sources/UntoldEngine/Physics/PhysicsBackendRegistry.swift — PhysicsBackendPlugin + manifest + validator following the render-extension plugin conventions (namespaced IDs, exact API-version match, backend ID inside plugin namespace); single-slot registry with install/replace/reject semantics, uninstall, failure introspection, and a lockForRuntime() seam for engine creation (wired in PR B).
• ColliderComponent and RigidBodyComponent in Components.swift, with cleanup handlers in RegistrationSystem.swift (handler ID physicsBody).
• 15 unit tests covering validation rejections, install/replace/uninstall, second-backend conflict, runtime lock, and inert default implementations.

Follow-ups per the plan: PR B (default backend wrap + PhysicsCoordinator + runFrame seam), PR C (event sink + USC OnCollision wiring), PR D (raycast facade).

Test plan

• swift test --filter PhysicsBackendRegistryTests — 15/15 pass
• ComponentRegistryTest (enforces cleanup handlers for every component in Components.swift) and ECSTests pass
• Engine builds with zero new warnings; no existing file touched beyond the two additive edits

[miogds]

Superseded by upstream untoldengine#1123 (merged as 260b23f with the review-revised layout), which is now in this fork's develop via the upstream sync.