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>
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