feat: include git hash in ntfy build notifications + MTU PRD

ntfy messages now show: "WZP Linux [abc1234] ready!" and
"WZP Android [abc1234] done! APK: url" so you can verify which
commit was built without checking relay version remotely.

Also added PRD-mtu-discovery.md for QUIC Path MTU Discovery.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

scripts/build-linux-docker.sh

@@ -114,7 +114,8 @@ docker run --rm \
 URL=$(curl -s -F "file=@/tmp/wzp-linux-x86_64.tar.gz" -H "Authorization: $rusty_auth_token" "$rusty_address")
 if [ -n "$URL" ]; then
     echo "UPLOAD_URL=$URL"
-    notify "WZP Linux x86_64 binaries ready! $URL"
+    GIT_HASH=$(cd /build/source && git rev-parse --short HEAD 2>/dev/null || echo unknown)
+notify "WZP Linux x86_64 [$GIT_HASH] ready! $URL"
     echo ">>> Done! Binaries at: $URL"
 else
     notify "WZP Linux build FAILED - upload error"