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/PHASES.md

@@ -102,6 +102,29 @@ where available. PLI + jitter validated on the desktop (induced-loss probe);
 hardware decode validated as clean-fallback on the desktop, hardware path
 pending a run on the Pi.
 
+## Phase 8 — Android Receiver App
+
+**Goal**: a native Android receiver app (Kotlin) so a phone can act as the
+second receiver — screen on, USB-C DP-alt-mode to HDMI. The app speaks the
+existing signaling + RTP protocol; no C++ changes.
+
+- [x] 8.1 Gradle project builds (`android/`, AGP 9 built-in Kotlin, no
+      androidx; `gradle :app:assembleDebug`)
+- [x] 8.2 Kotlin sources + JVM unit tests (rtp framing, jitter buffer,
+      depacketizer — 25 tests green)
+- [x] 8.3 Signaling + NSD validated on device (offer → answer over the
+      network; mDNS registration via the API-36 RegistrationListener)
+- [x] 8.4 MediaCodec decode + Surface render validated on device (in-band
+      SPS sizing, letterbox fit)
+- [x] 8.5 End-to-end on a Fairphone 6 (Android 16): streaming, letterboxed
+      fullscreen render, PLI keyframe recovery on Wi-Fi loss
+- [x] 8.6 Docs: README receiver section, RUNBOOK build/install/test,
+      PHASES + MEMORY updates
+
+**Validation**: `gradle :app:testDebugUnitTest` green; live session
+`--send --target window --peer <phone>:5005` rendered fullscreen on the
+phone (and HDMI via DP-alt-mode) with loss recovery.
+
 ## Current phase
 
-Phase 7 — Resilience and Polish.
+Phase 8 — Android Receiver App (complete).