fix: prevent SIGSEGV crash when Stop Recording clicked after mic replug

[jatinkrmalik]

Problem

@slatlasdev is hitting a daily crash:

1. Start recording
2. Replug/change microphone while stuck
3. App auto-recovers (engine resets, appStatus → .idle)
4. User clicks Stop Recording in the popover
5. App crashes — EXC_BAD_ACCESS (SIGSEGV), KERN_INVALID_ADDRESS

Root Cause

The crash trace from the IPS report (incident 48DBB805-253C-4F8B-9DCE-D6D389E10CE7, Sam Leatherdale, 2026-04-23):

SerialExecutor._isSameExecutor
SerialExecutor.isMainExecutor.getter
MainActor.assumeIsolated(_:file:line:)   ← faults here (EXC_BAD_ACCESS)
_ButtonGesture.internalBody.getter
PrimitiveButtonGestureCallbacks.dispatch

SwiftUI's _ButtonGesture internally wraps button action closures with MainActor.assumeIsolated { }. When a bare Task { } is used inside a Button action, the Swift 6 runtime synthesises an implicit assumeIsolated hop to validate the current executor. On macOS 26, if the view is being torn down or re-evaluated at the same moment the gesture fires — exactly what happens when onAudioDeviceChanged mutates @Published state and removes the Stop button while the user's click is mid-dispatch — the executor reference the runtime reads has already been freed, causing the SIGSEGV.

Why mic replug reliably triggers it:

• AVAudioEngineConfigurationChange fires → handleAudioConfigurationChange tears down engine, dispatches onAudioDeviceChanged on main
• onAudioDeviceChanged immediately sets isRecording = false + appStatus = .idle
• SwiftUI re-renders statusSection, removing the Stop Recording button from the view tree
• The user's in-flight click dispatches through the now-stale _ButtonGesture node → crash

Fixes

1. Task { @MainActor in } in all view button actions (primary fix)

Adding @MainActor to the Task closure binds it directly to the main actor's serial executor at the call site, eliminating the implicit assumeIsolated wrapper that SwiftUI synthesises. This is the correct Swift 6 pattern for all Button actions that call @MainActor-isolated methods.

Files changed:

• MenuBarView.swift — Stop Recording button
• UpdateView.swift — Download & Install, Retry buttons
• OnboardingView.swift — onAppear startup, model select/download, test recording toggle
• SettingsView.swift — Load, Download & Load, force-download alert, Check for Updates buttons

2. Reset stale callback flags in AudioEngine.handleAudioConfigurationChange (secondary fix)

When the audio device changes mid-recording, the handler was already stopping the engine and resetting isCurrentlyRecording, but left silenceCallbackFired and maxDurationCallbackFired set. On the next recording session after replug, those stale true flags meant neither the silence detector nor the max-duration limiter would ever fire — so recordings could run indefinitely with no auto-stop. Both flags are now cleared alongside isCurrentlyRecording.

Testing

• ✅ All 163 existing unit tests pass (swift test)
• Manual reproduction: replug mic during recording → click Stop → no crash

References

• Crash: EXC_BAD_ACCESS / SIGSEGV, incident 48DBB805-253C-4F8B-9DCE-D6D389E10CE7
• macOS 26.4.1 (25E253), VocaMac 0.6.0, Apple Silicon (Mac16,7)