refactor: decompose TranscriptionEngine god object into focused managers

[Newarr]

Stacked on #5 — after the logging migration merges, this PR introduces the manager extraction.

What

Decomposes the 949-line TranscriptionEngine god object into 3 focused, testable managers:

Manager	Lines	Responsibility
ModelDownloadManager	219	Model download, loading, progress tracking
DeviceRoutingManager	333	CoreAudio listeners, device restart coordination
TranscriptionStreamCoordinator	374	Audio stream orchestration, transcribers

Result

• TranscriptionEngine: 949 → 551 lines (42% reduction)
• Clean separation of concerns with single-responsibility managers
• Public API preserved for SwiftUI compatibility
• Swift 6 strict concurrency throughout

Architecture

TranscriptionEngine (551 lines, facade)
├── ModelDownloadManager - model lifecycle
├── DeviceRoutingManager - device listeners/restarts  
├── TranscriptionStreamCoordinator - stream orchestration
└── Existing utilities (unchanged)

Key Changes

New Files

• ModelDownloadManager.swift - Extracted model loading/download logic
• DeviceRoutingManager.swift - Extracted CoreAudio device listeners
• TranscriptionStreamCoordinator.swift - Extracted stream orchestration
• Utils/Logging.swift - Centralized Log enum (from refactor: migrate diagLog to unified os.Logger #5)

TranscriptionEngine Simplification

• Delegates model loading to ModelDownloadManager
• Delegates device listeners to DeviceRoutingManager with async callbacks
• Delegates stream management to TranscriptionStreamCoordinator
• Maintains all @Observable properties for SwiftUI binding

Concurrency Fixes (from code review)

• Converted fire-and-forget callbacks to async/await
• Added bounded buffering .bufferingNewest(1) to AsyncStreams
• Removed nonisolated(unsafe) patterns in favor of proper Sendable
• Fixed diarization task lifecycle (proper cancel/await)

Testing

• swift build compiles with Swift 6 strict concurrency
• All 304 tests pass
• No public API changes — existing SwiftUI bindings work unchanged

Migration Path

This is a pure refactoring with no behavioral changes. The public API remains:

@Observable @MainActor final class TranscriptionEngine {
    var isRunning: Bool
    var assetStatus: String
    var lastError: String?
    var needsModelDownload: Bool
    var downloadProgress: Double?
    var audioLevel: Float
    var isMicMuted: Bool
    
    func start(locale:inputDeviceID:transcriptionModel:) async
    func stop()
    func finalize() async
    func restartMic(inputDeviceID:)
}