[Feature] Deliver physics backend events to subscribers and USC scripts

[miogds]

Summary

PR C of the narrowed phase-1 plan from discussion #1116 (follows #1123 and #1129): event delivery. Backends already buffer contact/trigger/activation events internally and hand them to the coordinator's sink on the frame thread after each fixed substep (drainEvents, from PR A's threading contract) — this PR makes that drain go somewhere. With no external backend installed nothing is ever dispatched: the built-in integrator has no collision detection, so the whole path stays dormant and runtime behavior is unchanged.

• Sources/UntoldEngine/Physics/PhysicsEvents.swift — subscription hub in the FrameEvents style: onContact/onTrigger/onActivation return the existing EventSubscription token; handlers run on the frame thread during the post-step drain, in backend delivery order; subscriptions can be created/cancelled from any thread (snapshot-under-lock dispatch, same reentrancy semantics as FrameEventDispatcher). droppedEventCount surfaces backend fixed-capacity buffer overflow as a running counter — never an allocation or error mid-step.
• PhysicsEventDispatchSink replaces the coordinator's inert placeholder sink and also wires USC: contactBegan fires the previously never-fired OnCollision on both entities, plus the builder's tagged form OnCollision:<name> when the other entity has an explicit name; triggerEntered/triggerExited fire OnTriggerEnter/OnTriggerExit the same way (both names already appear in USCInstruction's event-handler documentation). Firing respects gameMode, via the existing USCSystem.triggerEvent path — no USC internals changed.
• 9 new tests with an event-emitting mock backend: subscription dispatch and cancellation, dormancy without a backend, drop-counter accumulation across substeps, USC OnCollision (untagged, tagged-by-name, and phase-filtered — persisted/ended must not re-fire), trigger enter/exit scripts, and gameMode gating.

The only files touched are in Physics/ plus the two physics test files. Follow-up per the plan: PR D (raycast facade) completes phase 1.

Test plan

• swift test --filter PhysicsEventsTests — 9/9 pass
• PhysicsCoordinatorTests + PhysicsBackendRegistryTests — 30/30 pass
• Full UntoldEngineTests module: 1093 tests, no new failures (only the known env-dependent ExternalRenderExtensionPackageTests)
• make lint with pinned SwiftFormat 0.60.1 — clean