feat(apple): session audio — host playback + mic uplink, device pickers in Settings

Both directions of the audio plane, on CoreAudio's built-in Opus codec
(kAudioFormatOpus — no bundled libopus; OpusCodec.swift, round trip unit-tested):

- Playback: a drain thread pulls nextAudio() packets, decodes, and writes a priming
  jitter ring feeding an AVAudioSourceNode (~20 ms prefill, adaptive to the device's
  render quantum so large-buffer devices don't oscillate prime/dropout; a high-water
  clamp sheds stall backlog so one network hiccup can't permanently lag audio behind
  video; underrun re-primes — one dip, not sustained crackle).
- Mic: a second engine taps the input device, resamples to 48 kHz stereo, Opus-encodes
  20 ms chunks and sendMic()s them into the host's virtual PipeWire source. Permission
  via AVCaptureDevice (NSMicrophoneUsageDescription added to the Xcode target).
- Settings: Speaker + Microphone pickers (CoreAudio HAL enumeration, persisted by
  device UID — "System default" leaves the engine unpinned so it follows macOS device
  changes) and a "Send microphone" toggle (default on). Applies from the next session.
- Audio starts with streaming, never during the trust prompt (no host sound — and no
  mic uplink — before the user trusted the host); teardown stops audio before close().

Adversarial-review fixes baked in: stop() and the dangling mic-permission callback
share one lock+flag protocol (no hot mic with no owner), the connect-success handler
bails when the attempt was abandoned mid-handshake (no session/mic for a dead window),
SessionAudio gets a deinit backstop (a dropped instance can't pin the connection via
its drain thread), and the render scratch buffer is block-owned (was leaked per
session).

Verified live against the box: remote test decodes 100 host Opus packets to PCM and
the host opens its virtual mic on the first uplinked frame ("punktfunk/1 virtual mic
ready"); on-glass session runs with both engines up.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

clients/apple/Punktfunk.xcodeproj/project.pbxproj

@@ -257,6 +257,7 @@
 				INFOPLIST_KEY_CFBundleDisplayName = "Punktfunkempfänger";
 				INFOPLIST_KEY_LSApplicationCategoryType = "public.app-category.utilities";
 				INFOPLIST_KEY_NSHumanReadableCopyright = "";
+				INFOPLIST_KEY_NSMicrophoneUsageDescription = "Your microphone is streamed to the connected punktfunk host, where it appears as a virtual microphone.";
 				INFOPLIST_KEY_NSPrincipalClass = NSApplication;
 				LD_RUNPATH_SEARCH_PATHS = (
 					"$(inherited)",
@@ -283,6 +284,7 @@
 				INFOPLIST_KEY_CFBundleDisplayName = "Punktfunkempfänger";
 				INFOPLIST_KEY_LSApplicationCategoryType = "public.app-category.utilities";
 				INFOPLIST_KEY_NSHumanReadableCopyright = "";
+				INFOPLIST_KEY_NSMicrophoneUsageDescription = "Your microphone is streamed to the connected punktfunk host, where it appears as a virtual microphone.";
 				INFOPLIST_KEY_NSPrincipalClass = NSApplication;
 				LD_RUNPATH_SEARCH_PATHS = (
 					"$(inherited)",