refactor(#1023): extract containsWakePhrase, resolve wake model from selected engine

[lokhor]

Summary

Pure refactor — no behavioural changes. Extracts containsWakePhrase() from SherpaOnnxVoiceInputController.Companion into a shared top-level utility and promotes transcribeBlocking() to the VoiceInputController interface. Additionally, the wake-word verification engine now follows the user's selected online STT model instead of always using Zipformer.

Changes — Round 1: Extraction

File	Change
core/voice/.../WakeWordUtils.kt (new)	Top-level fun String.containsWakePhrase() — same regex, public visibility
core/voice/.../VoiceInputController.kt	Adds suspend fun transcribeBlocking(pcm: ShortArray): String? = null with doc comment
core/voice/.../SherpaOnnxVoiceInputController.kt	Removes companion containsWakePhrase; changes transcribeBlocking to override suspend fun ... String? (null on failure)
app/.../WakeWordService.kt	Calls voiceInputController.transcribeBlocking(pcm)?.containsWakePhrase() ?: false instead of hardcoded Sherpa field; drops unused injection
core/voice/.../SherpaOnnxVoiceInputControllerTest.kt	Removes the 5 containsWakePhrase tests + stale import
core/voice/.../WakeWordUtilsTest.kt (new)	Same 5 test cases against top-level function, MockK-free

Changes — Round 2: Model selection for wake verification

transcribeBlocking() previously hardcoded SherpaSttModelSpec.WAKE_VERIFICATION_DEFAULT (Zipformer). It now resolves the user's selected engine and uses it when it's an online recognizer (Zipformer/Paraformer), falling back to Zipformer for offline-only models (SenseVoice/Whisper).

File	Change
core/voice/.../SherpaOnnxVoiceInputController.kt	transcribeBlocking → resolveSpec() + Online check; added releaseWakeRecognizer() to prevent native memory leak on engine switch; ensureWakeRecognizerBlocking doc/log updated
feature/settings/.../VoiceViewModel.kt	Added resolveWakeEngineForDownload() helper; both download sites use it instead of hardcoding SherpaZipformer
core/voice/.../SherpaSttModelSpec.kt	WAKE_VERIFICATION_DEFAULT doc updated to reflect fallback role
docs/SPECIFICATION.md	Zipformer table row and wake-word verification section updated

Behaviour matrix

Selected engine	Wake verification model
Zipformer	Zipformer (same model, separate instance)
Paraformer	Paraformer (same model, separate instance)
SenseVoice	Zipformer (fallback — offline-only)
Whisper	Zipformer (fallback — offline-only)

Verification

• :core:voice:test ✓
• :app:testDebugUnitTest ✓
• assembleDebug ✓
• No stale references to SherpaOnnxVoiceInputController.Companion.containsWakePhrase or the removed field

Closes #1023

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