feat(apple/M2): opt-in background keep-alive (audio + video-drop + timeout)

Backgrounding a live session no longer freezes it when the user opts in: audio
keeps playing (UIBackgroundModes audio), the QUIC connection + pump stay live,
video decode is DROPPED, and a bounded timer auto-disconnects. Off by default.

- PunktfunkConnection.setVideoDropped/isVideoDropped: a tiny lock-guarded flag
  both pumps read every iteration. StreamPump (stage-1), Stage2Pipeline (VT +
  PyroWave) drain nextAU() for flow control but DISCARD the AU before any
  VideoToolbox/Metal work — the crash/jetsam-safe seam (no GPU off-screen).
- SessionModel.enterBackground(timeoutMinutes:) / exitBackground(): set the drop
  flag, mute the mic (privacy — SessionAudio.setMicMuted pauses the capture
  engine), arm a DispatchSourceTimer that disconnect(deliberate:false)s on fire
  (keeps host linger → fast late reconnect). exitBackground clears the flag and
  requestKeyframe()s; the pump's freeze gate auto-arms on the resumed
  frame-index gap so concealed frames are withheld until the IDR re-anchors.
  disconnect() cancels the timer + clears isBackgrounded.
- ContentView scenePhase driver (iOS): .background+streaming+setting →
  enterBackground; .active → exitBackground. scenePhase (not willResignActive)
  so Control-Center/app-switcher peeks don't start the timer.
- Settings → General (iOS-only keepAliveSection): toggle + 1/5/10/30 timeout;
  new keys backgroundKeepAlive (def off) / backgroundTimeoutMinutes (def 10).
- Info.plist: UIBackgroundModes [audio] + NSSupportsLiveActivities (for M3).

macOS swift build + swift test green (142 tests). The iOS-gated scenePhase
handler + settings section are not exercised by the macOS CI target (known §9
gap) — need on-glass verification (audio never gaps, video re-anchors <1s LAN,
timeout ends the session, phone-call audio-steal degrades gracefully).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

clients/apple/Config/Info.plist

@@ -19,6 +19,19 @@
 	<array>
 		<string>_punktfunk._udp</string>
 	</array>
+	<!-- Background keep-alive (opt-in, iOS/iPadOS): the ONLY sanctioned way to keep the long-lived
+	     QUIC socket + pump-thread set alive while backgrounded is the audio background mode, backed
+	     by the session's real, audible remote audio (AVAudioEngine keeps rendering). Video decode is
+	     dropped; a bounded timer auto-disconnects. Never silence-as-keepalive (App Review 2.5.4).
+	     tvOS ignores/tolerates the key; macOS is not gated by it. -->
+	<key>UIBackgroundModes</key>
+	<array>
+		<string>audio</string>
+	</array>
+	<!-- Live Activities (iOS/iPadOS): the Lock-Screen / Dynamic-Island session surface. Updated
+	     locally (pushType nil) from the alive app process — no aps-environment. tvOS/macOS ignore it. -->
+	<key>NSSupportsLiveActivities</key>
+	<true/>
 	<!-- Deep links: punktfunk://connect/<host-uuid>[?launch=<GameEntry.id>]. Emitted by the
 	     launcher widget and Siri/Shortcuts; routed by ContentView.onOpenURL into the existing
 	     connect path. Shared across all three targets (tvOS/macOS accept it harmlessly). -->