feat: Android engine + Kotlin API for direct 1:1 calling

Rust engine:
- start_signaling(): persistent _signal connection, presence registration
- Signal recv loop: handles DirectCallOffer, CallRinging, CallSetup, Hangup
- New CallState variants: Registered, Ringing, IncomingCall
- Stats expose incoming_call_id, incoming_caller_fp, incoming_caller_alias, sas_code
- New EngineCommands: PlaceCall, AnswerCall, RejectCall

JNI bridge:
- nativeStartSignaling(relay, seed, token, alias)
- nativePlaceCall(targetFp)
- nativeAnswerCall(callId, mode)

Kotlin API (WzpEngine.kt):
- startSignaling(relay, seed, token, alias)
- placeCall(targetFingerprint)
- answerCall(callId, mode) — 0=Reject, 1=AcceptTrusted, 2=AcceptGeneric

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

crates/wzp-android/src/commands.rs

@@ -12,4 +12,13 @@ pub enum EngineCommand {
     ForceProfile(QualityProfile),
     /// Stop the call and shut down the engine.
     Stop,
+    /// Place a direct call to a fingerprint (requires signal connection).
+    PlaceCall { target_fingerprint: String },
+    /// Answer an incoming direct call.
+    AnswerCall {
+        call_id: String,
+        accept_mode: wzp_proto::CallAcceptMode,
+    },
+    /// Reject an incoming direct call.
+    RejectCall { call_id: String },
 }