fix: federation presence dedup, stale cleanup, and Android SIGSEGV crash

- Deduplicate remote participants by fingerprint in all merge sites
  (canonical == raw room name caused double-lookup, doubling every remote participant)
- GlobalRoomInactive now propagates updated participant list to other peers
  (hub relay B was not informing A when C's participants left)
- Add 15-second stale presence sweeper that purges remote participants
  from peers that stop sending data (safety net for QUIC timeout delays)
- Add @Synchronized to WzpEngine.getStats/stopCall/destroy to prevent
  TOCTOU race between stats polling coroutine and engine teardown (SIGSEGV)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

android/app/src/main/java/com/wzp/engine/WzpEngine.kt

@@ -53,6 +53,7 @@ class WzpEngine(private val callback: WzpCallback) {
     }
 
     /** Stop the active call. Safe to call when no call is active. */
+    @Synchronized
     fun stopCall() {
         if (nativeHandle != 0L) {
             nativeStopCall(nativeHandle)
@@ -76,6 +77,7 @@ class WzpEngine(private val callback: WzpCallback) {
      *
      * @return JSON-serialised [CallStats], or `"{}"` if the engine is not initialised.
      */
+    @Synchronized
     fun getStats(): String {
         if (nativeHandle == 0L) return "{}"
         return try {
@@ -95,6 +97,7 @@ class WzpEngine(private val callback: WzpCallback) {
     }
 
     /** Destroy the native engine and free all resources. The instance must not be reused. */
+    @Synchronized
     fun destroy() {
         if (nativeHandle != 0L) {
             nativeDestroy(nativeHandle)