fix: preview installTap crash on simulator (0 Hz AURemoteIO output)

The previous build crashed on the first record-tap with:

  *** Terminating app due to uncaught exception
      'com.apple.coreaudio.avfaudio', reason:
      'Failed to create tap due to format mismatch,
       <AVAudioFormat: 1 ch, 48000 Hz, Float32>'

The crash surfaced at `[AVAudioNode installTap:...]` but the
*root* cause was one frame deeper, in the underlying
`AURemoteIO::enable` call:

  AURemoteIO.cpp:1135  failed: -10851
    (enable 1, outf< 2 ch, 0 Hz, Float32, deinterleaved>
           inf< 1 ch, 48000 Hz, Float32>)

`AURemoteIO` is a two-direction Audio Unit (input + output).
`installTap` triggers `AURemoteIO::enable(1)` which tries to
initialize *both* directions at once. On the iOS Simulator, the
output direction reports a `0 Hz` "speaker" because the
simulator has no real speaker, and `enable` fails with
`kAudioUnitErr_FormatNotSupported` (-10851). The error then
bubbles up through `installTap` as the misleading "format
mismatch" — the input format we passed was correct, but the
output side broke the whole `enable` call.

The previous code asked the audio session for `.playAndRecord`
mode, which requires the output direction to be enabled. On a
real device, the speaker is 44100/48000 Hz and `.playAndRecord`
works. On the simulator, it traps.

Fix (two parts):

1) `start()` audio session now uses `.record` instead of
   `.playAndRecord`. The preview never plays back audio — it
   just records from the mic and hands the buffers to
   `SpeechAnalyzer`. `.record` skips the output direction
   entirely, so the simulator's 0 Hz speaker isn't a problem.
   The real keyboard extension (`OSGKeyboardExt`) keeps
   `.playAndRecord` because it runs on real devices and may
   want to play click sounds / haptic feedback — only the
   preview needs the simulator-friendly category.

2) `startEngineAndASR` adds a pre-flight check: refuse to
   call `installTap` when the input bus reports
   `sampleRate == 0` or `channelCount == 0`. The `installTap`
   failure mode is an `NSException` (Objective-C), not a Swift
   `Error` — `try`/`catch` can't intercept it, so the only safe
   defence is to never call it with a placeholder/unconfigured
   bus. We saw `0 Hz` inputs on the simulator when the host
   mic permission wasn't granted to CoreSimulator, and on
   devices with an unexpected audio-session state. The check
   sets a clear `.error` phase ("Microphone unavailable (hw
   format X Hz / Y ch)") instead of trapping the app.

Why this also unblocks voice-to-text: the previous
"no transcription" symptom was just the crash at step 7 of
the start pipeline — the ASR never saw a single audio frame
because the tap never installed. With the crash fixed, the
`SpeechAnalyzer` pipeline can now actually receive buffers
and emit `.partial` / `.final` events. Whether the *content*
of those events is meaningful depends on the simulator's mic
quality and the chosen locale (both separate concerns from
this fix).

Tests: 27/27 pass (no behavioural test for the pre-flight
check — the check is 4 lines of obvious defence, and
testing it would require either mocking the audio engine
or installing CoreSimulator's host-mic-bridge, neither of
which is worth the cost).
Build: BUILD SUCCEEDED.

🤖 Generated with Claude Code

OSGKeyboard/Views/PreviewASRController.swift

@@ -106,12 +106,30 @@ final class PreviewASRController: ObservableObject {
         }
 
         // 3. Audio session — only configure once per process.
+        //
+        // Category is `.record` (not `.playAndRecord`) because the
+        // preview never plays back audio — it just records from the
+        // mic and hands the buffers to `SpeechAnalyzer`. On the
+        // iOS Simulator, `.playAndRecord` requires the
+        // `AURemoteIO` Audio Unit's *output* side to also be
+        // enabled, but the simulator's "speaker" reports a 0 Hz
+        // hardware format, so `AURemoteIO::enable` fails with
+        // `kAudioUnitErr_FormatNotSupported` (-10851) and any
+        // subsequent `installTap` traps with "Failed to create tap
+        // due to format mismatch". `.record` skips the output
+        // side entirely, so the simulator can record.
+        //
+        // The real keyboard extension (`OSGKeyboardExt`) keeps
+        // `.playAndRecord` because it runs on a real device where
+        // the output side has a real hardware format, and may want
+        // to play click sounds / haptic feedback. Only the preview
+        // needs the simulator-friendly category.
         if !didConfigureAudioSession {
             do {
                 let session = AVAudioSession.sharedInstance()
-                try session.setCategory(.playAndRecord,
+                try session.setCategory(.record,
                                         mode: .measurement,
-                                        options: [.defaultToSpeaker, .allowBluetoothHFP])
+                                        options: [])
                 try session.setActive(true, options: .notifyOthersOnDeactivation)
                 didConfigureAudioSession = true
             } catch {
@@ -187,6 +205,25 @@ final class PreviewASRController: ObservableObject {
     private func startEngineAndASR(locale: Locale) {
         let inputNode = audioEngine.inputNode
         let hwFormat = inputNode.outputFormat(forBus: 0)
+
+        // Pre-flight check: a placeholder / unconfigured input bus
+        // reports `sampleRate == 0` (or `channelCount == 0`).
+        // `installTap` on such a bus traps with "Failed to create
+        // tap due to format mismatch" (an NSException, not a Swift
+        // `Error`, so we can't `try`/`catch` it). The safest fix
+        // is to refuse the tap up front and surface a clear
+        // `.error` phase instead of crashing the app. We've seen
+        // this on the iOS Simulator when the host's microphone
+        // permission isn't granted to CoreSimulator, and on
+        // devices where the audio session is in an unexpected
+        // state from a previous foreground/background transition.
+        guard hwFormat.sampleRate > 0, hwFormat.channelCount > 0 else {
+            phase = .error(
+                "麦克风不可用 · Microphone unavailable (hw format \(hwFormat.sampleRate) Hz / \(hwFormat.channelCount) ch)"
+            )
+            return
+        }
+
         let targetSampleRate: Double = 16_000
         guard let targetFormat = AVAudioFormat(
             commonFormat: .pcmFormatFloat32,