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.

docs/RUNBOOK.md

@@ -209,6 +209,55 @@ The headless equivalent runs as part of `meson test` (`udp loopback` test):
 synthetic frames → encode → packetize → localhost UDP → depacketize →
 decode, no portal or window involved.
 
+## Android receiver app (Phase 8)
+
+Toolchain: JDK 21, system `gradle` (no wrapper), Android SDK with android-36.
+AGP 9 has **built-in Kotlin** — do not apply `org.jetbrains.kotlin.android`
+and do not use `kotlinOptions {}`.
+
+```sh
+cd android
+gradle :app:assembleDebug            # -> app/build/outputs/apk/debug/app-debug.apk
+gradle :app:testDebugUnitTest        # 25 JVM tests (rtp, jitter, depacketizer)
+```
+
+Device test (validated on a Fairphone 6, Android 16 / API 36):
+
+```sh
+adb install -r app/build/outputs/apk/debug/app-debug.apk
+adb shell am start -n screen_cast.receiver/screen_cast.ReceiverActivity
+# activity FQN is screen_cast.ReceiverActivity (namespace), not
+# screen_cast.receiver.ReceiverActivity
+adb shell uiautomator dump /sdcard/ui.xml && adb shell cat /sdcard/ui.xml   # status text
+adb exec-out screencap -p > phone.png                                      # visual check
+```
+
+On the sender: `screencast --send --peer <phone-ip>:5005` (add `--target
+window` to pick a specific window; the portal's monitor source picks the
+first output on multi-monitor Hyprland/GTK-portal setups).
+
+Platform quirks found while validating (Android 16 / API 36):
+
+- `android.permission.INTERNET` is required — NsdService rejects
+  registration without it.
+- `DatagramSocket.localPort` gives the bound port; `.port` is -1 for
+  unconnected datagram sockets, and `.localAddress` is an `InetAddress`
+  (Inet6Address on Android), not an `InetSocketAddress`.
+- `MediaCodec`: pass the output Surface to `configure()` (`setOutputSurface`
+  is illegal after configure); call `start()`; render with
+  `releaseOutputBuffer(index, render=true)`; the C2 AVC decoder requires a
+  concrete width/height at configure (use a placeholder — the in-band SPS
+  reconfigures it).
+- `MediaFormat.format()` / `KEY_MIME_TYPE` are not in the API-36 public
+  surface.
+- NSD: API 36 replaced `ResolutionListener` registration with
+  `registerService(info, flags, RegistrationListener)`; the app falls back
+  to reflection on older devices.
+- The C2 decoder scales its output to the Surface (`android._video-scaling`)
+  — letterbox by **sizing the TextureView to the video aspect** (centered
+  on a black window background), not with a transform matrix (double scale).
+- mDNS does not cross subnets; `--peer` is the reliable path.
+
 ## Formatting
 
 ```sh