Add Stage 4 runtime proxy slots (max 4)
Introduce a controller-local runtime proxy slot manager (ProxyActorSlot) and g_proxySlots map capped at kMaxRuntimeProxyActors = 4. Transition the proxy flow from a single active runtime proxy to per-remote-player slots: sorted remote snapshots, per-slot pre-spawn candidate snapshots, PlaceAtMe-backed spawn attempts (one in-progress spawn globally), isolation/settle/promotion of placed refs into slot.proxyHandle, per-slot movement smoothing/hold logic, and validation/restore of handles. Preserve the single placed fallback reference for one selected player and keep slots session-sticky (no reuse/despawn yet). Update docs (architecture.md, dev-log.md) and header/source (F4TProxyActorController.{h,cpp}) to reflect Stage 4 behavior and diagnostics.
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