docs: document the Android receiver app (Phase 8)

README: phone-as-second-screen section (build, install, sender usage,
multi-monitor caveat). RUNBOOK: Android build/test procedure, device-test
commands, and the API-36 platform quirks found during validation.
PHASES: Phase 8 milestones, all validated. MEMORY: current phase state
and the hard-won Android/portal lessons.

.agents/MEMORY.md

@@ -1,10 +1,33 @@
 # Project Memory — screen_cast
 
-Last updated: Phase 6 (discovery + signaling) complete and validated over
-loopback; current phase is Phase 7.
+Last updated: Phase 8 (Android receiver app) complete and validated on a
+Fairphone 6; all prior phases done.
 
 ## Project state
 
+- **Phase 8 done: Android receiver app** (`android/`, Kotlin, minSdk 30,
+  app id `screen_cast.receiver`): phone as second receiver (screen → HDMI via
+  USB-C DP-alt-mode). Speaks the existing signaling+RTP protocol — no C++
+  changes. AGP 9 **built-in Kotlin** (no kotlin plugin, no `kotlinOptions`;
+  Kotlin targets `compileOptions`, Java 17); no androidx (framework +
+  org.json + JUnit). 25 JVM tests green. Validated end-to-end on a
+  Fairphone 6 (API 36): offer/answer → MediaCodec → fullscreen letterboxed
+  render; PLI recovery on real Wi-Fi loss. Hard-won API-36 quirks (in
+  RUNBOOK): `INTERNET` permission is REQUIRED for NsdService;
+  `DatagramSocket.localPort` (`.port` is -1 unconnected; `.localAddress` is
+  Inet6Address, not InetSocketAddress); pass the Surface to
+  `MediaCodec.configure()` + `start()`; render via
+  `releaseOutputBuffer(render=true)`; C2 AVC needs a concrete size at
+  configure (in-band SPS reconfigures); `MediaFormat.format()`/
+  `KEY_MIME_TYPE` not public in API 36; NSD `RegistrationListener` replaced
+  `ResolutionListener` (reflection fallback for older devices);
+  **`android._video-scaling`: C2 scales output to the Surface** → letterbox
+  by sizing the TextureView to the video aspect, NOT a transform matrix
+  (double scale). Sender-side: Hyprland + GTK portal's `--target monitor`
+  picks the FIRST output (eDP-1, wrong display) — use `--target window`
+  (hyprland-share-picker). Activity FQN for `am start`:
+  `screen_cast.receiver/screen_cast.ReceiverActivity` (Kotlin package =
+  namespace `screen_cast`).
 - **Phase 6 done**: Avahi mDNS discovery (`_screencast._tcp` — receiver
   announces its signaling port via a threaded-poll Avahi client; senders
   browse+resolve) plus JSON session signaling (offer/answer) over TCP with