refactor: drop Qwen3 CoreML ASR, add local-engine cloud polish toggle

Rolls back the v0.2.0 Qwen3 CoreML on-device ASR stack and replaces the
'local engine' UX with iOS 26 SpeechAnalyzer + DictationTranscriber only.

The 'Cloud polish after ASR' toggle (ProviderConfig.localModeCloudPolishEnabled)
lets users opt into a post-ASR DeepSeek round-trip from the local engine.
Defaults to off so the local engine stays genuinely local. New PolishError.missingAPIError
surfaces an inline 'fill in your key' warning when the toggle is on but the
Keychain is empty. DeepSeek preset default model bumped to deepseek-v4-flash.

Deleted:
  - OSGKeyboard/ThirdParty/Qwen3Speech/ (74 files, ~16k LoC)
  - OSGKeyboard/Services/ModelManager.swift (492)
  - OSGKeyboard/Services/OnDeviceModelWarmup.swift (197)
  - OSGKeyboard/Services/Qwen3ASRService.swift (257)
  - OSGKeyboard/Services/ModelDownloadSourcePicker.swift (126)
  - OSGKeyboard/Views/OnDeviceModelsView.swift (184)
  - OSGKeyboard/Views/DownloadConfirmSheet.swift (96)
  - OSGKeyboardShared/Models/OnDeviceModel.swift (140)
  - OSGKeyboardShared/Services/OnDeviceModelStatus.swift (104)
  - Qwen3ASRServiceProvider registration in OSGKeyboardApp
  - Qwen3Speech package declaration in project.yml
  - 5 .qwen3ASR enum / branch reference sites in HomeView, OnboardingView,
    LocalEngineSettingsRows, FlowSessionManager, ASRService, EngineServiceLabel
  - Two pre-existing Swift 6 strict-concurrency errors in
    LiveDictationController + FlowSessionManager (the weak [weak self] in
    detached-task MainActor.run blocks) that were blocking clean builds

Added:
  - LocalModelsGroup: 'Built-in iOS SpeechAnalyzer' badge + 'Cloud polish
    after ASR' Switch toggle
  - PolishingService: honour localModeCloudPolishEnabled; new .missingAPIKey
    error case with localised warning
  - AppGroupStore.localModeCloudPolishEnabled (mirrored into App Group
    so the keyboard extension honours the toggle during live dictation)
  - SettingsView: show provider/api sections when local-mode cloud polish
    is on so the user can paste a DeepSeek key
  - FlowSessionManager: route through PolishingService for local + polish-on
    flow; translate missingAPIKey into a polished warning
  - KeyboardViewController: handle PolishingService.PolishError.missingAPIKey
    in the keyboard-side live polish path
  - CHANGELOG v0.2.1: documents the rollback + new toggle
  - README.md / README.zh.md: engine matrix section, data flow note

Verified: xcodebuild -scheme OSGKeyboard -destination 'generic/platform=iOS Simulator'
build succeeds under SWIFT_STRICT_CONCURRENCY=complete.

OSGKeyboardShared/Services/KeyboardState.swift

@@ -77,10 +77,15 @@ public final class KeyboardState: ObservableObject {
     /// Mirrored from `ProviderConfig.localASRBackend` for UI display
     /// and for `state` consumers that want a single source of truth.
     @Published public var localASRBackend: LocalASRBackend = .speechAnalyzer
-    /// `false` when the local engine needs on-device models that are
-    /// not yet downloaded (mirrored from App Group by the extension).
+    /// v0.2.0: kept for source compatibility with the previous Qwen3
+    /// CoreML local engine. Always `true` now — iOS `SpeechAnalyzer`
+    /// ships with iOS 26 and has no per-user weights to download or
+    /// preload. Existing read sites will see `true` and behave the
+    /// same as the "stack ready" branch did.
     @Published public var localModelsReady: Bool = true
-    /// `true` when host app has preloaded Qwen weights into memory.
+    /// v0.2.0: kept for source compatibility with the previous Qwen3
+    /// CoreML local engine. Always `false` now — there are no weights
+    /// for the host app to preload.
     @Published public var localModelsLoaded: Bool = false
 
     /// Convenience shorthand used by the pipeline and views.