Phase 1: dual-track audio capture → mixed-mono 16 kHz WAV + mic VAD

AudioRecorder captures system audio (ScreenCaptureKit) + mic (AVAudioEngine) on a
single serial ioQueue, one shared monotonic t0, time-driven writers (pad gaps /
trim overlaps) so tracks stay aligned, and an energy mic-VAD for 'self' spans.
AudioMixer sums the aligned tracks into mixed_mono_16k.wav. SessionController
drives a serialized start/stop state machine, writes the session folder +
self_vad.json, exposes live level meters, and finalizes on quit.

Hardening from review: ioQueue single-domain (no races), stop() never hangs
(mic-first teardown + bounded stopCapture), layout-agnostic mic deep-copy,
discard-only video output to keep SCStream alive, VAD lockstep on committed
frames, stable signing team in project.yml, single-instance enforcement.

Ten31Transcripts/App/AppDelegate.swift

@@ -6,5 +6,30 @@ final class AppDelegate: NSObject, NSApplicationDelegate {
         // LSUIElement in Info.plist already enforces this; set it explicitly too
         // so behavior is unambiguous regardless of how the app is launched.
         NSApp.setActivationPolicy(.accessory)
+        terminateOtherInstances()
+    }
+
+    /// Single-instance: a fresh launch (e.g. each Xcode ⌘R) terminates any older
+    /// copies so you never end up with two menu-bar icons.
+    private func terminateOtherInstances() {
+        guard let bundleID = Bundle.main.bundleIdentifier else { return }
+        let me = NSRunningApplication.current.processIdentifier
+        for app in NSRunningApplication.runningApplications(withBundleIdentifier: bundleID)
+        where app.processIdentifier != me {
+            app.terminate()
+        }
+    }
+
+    /// If a recording is in progress when the user quits, finalize it (flush WAV
+    /// headers) before the process exits, so the session isn't corrupted.
+    func applicationShouldTerminate(_ sender: NSApplication) -> NSApplication.TerminateReply {
+        guard let controller = SessionController.shared, controller.isBusy else {
+            return .terminateNow
+        }
+        Task { @MainActor in
+            await controller.prepareForTermination()
+            NSApp.reply(toApplicationShouldTerminate: true)
+        }
+        return .terminateLater
     }
 }