docs: record real two-machine validation of Phase 6 on the Pi

The full chain — mDNS discovery, signaling negotiation, RTP over
Wi-Fi, software H.264 decode, and fullscreen KMSDRM letterboxed
rendering — is confirmed working on a real two-machine setup
(desktop -> Raspberry Pi Zero 2 W headless receiver). The probe
gradient appeared on the Pi's display.

docs/PHASES.md

@@ -73,9 +73,12 @@ session.
 - [x] Extend CLI with `--peer` and `--discover`.
 
 **Validation**: two peers connect without hard-coded IP addresses.
-Validated end-to-end over loopback: discovery finds the announced receiver,
-the sender negotiates a session over the signaling channel, and streams to
-the negotiated endpoint over both IPv4 and IPv6.
+Validated over loopback AND on real two-machine hardware: a Raspberry
+Pi Zero 2 W (Wi-Fi, headless KMSDRM, systemd autostart) was discovered,
+negotiated with, and streamed to from the desktop; the receiver rendered
+a synthetic feed fullscreen on its display. On that network the LAN's
+6to4 addresses are unreachable between peers, so senders try all
+discovered addresses in reachability order (private IPv4 first).
 
 ## Phase 7 — Resilience and Polish