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/Sources/PunktfunkKit/Audio/SessionAudio.swift

@@ -180,6 +180,23 @@ public final class SessionAudio {
         }
     }
 
+    /// Background keep-alive: silence the mic uplink while backgrounded (privacy — no room audio
+    /// leaves the device) and restore it on return. Pauses/resumes the capture engine; a no-op when
+    /// there's no uplink (playback-only / tvOS / mic disabled). The audio SESSION stays active for
+    /// background playback, so iOS may keep showing the recording indicator until a full reconfigure
+    /// — this stops the actual capture, which is the privacy-relevant part. Main thread.
+    public func setMicMuted(_ muted: Bool) {
+        stateLock.lock()
+        let capture = captureEngine
+        stateLock.unlock()
+        guard let capture else { return }
+        if muted {
+            capture.pause()
+        } else if !flag.isStopped {
+            try? capture.start()
+        }
+    }
+
     // MARK: - Playback (host → speaker)
 
     private func startPlayback(speakerUID: String) {

clients/apple/Sources/PunktfunkKit/Connection/PunktfunkConnection.swift

@@ -534,6 +534,23 @@ public final class PunktfunkConnection {
         _ = punktfunk_connection_request_keyframe(h)
     }
 
+    /// Background-keep-alive video drop (opt-in). While true, both video pumps keep DRAINING
+    /// `nextAU()` (so QUIC flow control and host pacing stay healthy) but DISCARD each AU before any
+    /// VideoToolbox/Metal decode or render — the crash/jetsam-safe way to hold a backgrounded
+    /// session (audio keeps rendering; no GPU work off-screen). Set on `SessionModel.enterBackground`,
+    /// cleared on `exitBackground` (which then requests a fresh IDR; the pump's re-anchor gate
+    /// auto-arms on the resumed frame-index gap). Its own tiny lock — read on the pump thread every
+    /// iteration, written on the main actor; never contends the ABI/plane locks.
+    private let videoDropLock = NSLock()
+    private var videoDropped = false
+    public var isVideoDropped: Bool {
+        videoDropLock.lock(); defer { videoDropLock.unlock() }
+        return videoDropped
+    }
+    public func setVideoDropped(_ dropped: Bool) {
+        videoDropLock.lock(); videoDropped = dropped; videoDropLock.unlock()
+    }
+
     /// Feed each received AU's `frameIndex` (in receive order) so the client recovers from loss with a
     /// cheap reference-frame invalidation instead of always paying for a full IDR. On a forward gap —
     /// a `frameIndex` jump means the intervening frames were lost and the following AUs reference a

clients/apple/Sources/PunktfunkKit/Video/Stage2Pipeline.swift

@@ -432,6 +432,15 @@ public final class Stage2Pipeline {
             while alive, !token.isStopped {
                 alive = autoreleasepool { () -> Bool in
                 do {
+                    // Background keep-alive: drain one AU (flow control + host pacing) and discard it
+                    // BEFORE any VideoToolbox decode or Metal render — no GPU work off-screen. The
+                    // decoder session is left intact; exitBackground requests a fresh IDR and the
+                    // re-anchor gate arms on the resumed frame-index gap so concealed frames are
+                    // withheld until it lands.
+                    if connection.isVideoDropped {
+                        _ = try connection.nextAU(timeoutMs: 100)
+                        return true
+                    }
                     // Loss recovery (the primary path). The reassembler drops unrecoverable AUs and the
                     // decoder conceals the reference-missing deltas — often WITHOUT an error callback —
                     // so key off the drop count climbing, then keep asking (awaitingIDR) until a fresh
@@ -687,6 +696,13 @@ public final class Stage2Pipeline {
             while alive, !token.isStopped {
                 alive = autoreleasepool { () -> Bool in
                     do {
+                        // Background keep-alive: drain + discard before the Metal wavelet decode
+                        // (PyroWave is all-intra, so the resumed frame heals on its own — no IDR
+                        // request needed, just no GPU work off-screen).
+                        if connection.isVideoDropped {
+                            _ = try connection.nextAU(timeoutMs: 100)
+                            return true
+                        }
                         guard let au = try connection.nextAU(timeoutMs: 100) else { return true }
                         onFrame?(au)
                         if let newest = newestIndex,

clients/apple/Sources/PunktfunkKit/Video/StreamPump.swift

@@ -63,6 +63,14 @@ final class StreamPump {
             while alive, !token.isStopped {
                 alive = autoreleasepool { () -> Bool in
                 do {
+                    // Background keep-alive: drain one AU to keep QUIC flow control + host pacing
+                    // healthy, then discard it BEFORE any decode/enqueue — no VideoToolbox/Metal work
+                    // off-screen. Skips all recovery/gate bookkeeping too; exitBackground requests a
+                    // fresh IDR and the re-anchor gate re-arms on the resumed frame-index gap.
+                    if connection.isVideoDropped {
+                        _ = try connection.nextAU(timeoutMs: 100)
+                        return true
+                    }
                     // Loss recovery (the primary path). Under the host's infinite GOP the only
                     // recovery keyframe is one we request. The reassembler drops unrecoverable AUs
                     // (framesDropped); the decoder then *conceals* the reference-missing deltas — a