feat(android): native receiver app (Kotlin, minSdk 30)

Phone-as-receiver for the USB-C -> HDMI mirroring use case. Speaks the
existing signaling + RTP wire protocol, so the C++ sender needs no
changes. AGP 9 built-in Kotlin (no separate plugin), no androidx.

- rtp/: RtpHeader, RtpPacket, JitterBuffer (16 pkt / 60 ms, straggler
  discard), H264Depacketizer (single-NAL + FU-A, Annex-B out) — JVM tests
- signaling/: newline-JSON server (offer -> answer, PLI, 500 ms rate cap)
- decode/H264Decoder: MediaCodec -> Surface; size from in-band SPS (offer
  is 0x0); surface passed to configure(); releaseOutputBuffer(render)
- pipeline/ReceiverPipeline: NSD advertise (API-36 RegistrationListener
  + reflection fallback), UDP RTP, jitter, decode, PLI recovery
- ReceiverActivity: fullscreen TextureView letterboxed by sizing the
  view to the video aspect (C2 scales output to the surface)

Validated end-to-end on a Fairphone 6 (API 36): offer/answer, first
frame, letterbox fit, PLI on loss.

android/.gitignore

@@ -0,0 +1,7 @@
+.gradle/
+.kotlin/
+build/
+local.properties
+.idea/
+captures/
+.cxx/