Stop invalidating ContentView on streaming partial transcription updates

[NathanDrake2406]

What this changes

ASRService.partialTranscription moves from @Published to a private CurrentValueSubject<String, Never> exposed to subscribers via an AnyPublisher. The value getter and the single Combine consumer (MenuBarManager) are updated accordingly. No other @Published properties on ASRService are affected.

Why

During streaming ASR, partialTranscription is written 1-5 times per second depending on the model (parakeetRealtime runs at 0.2s intervals, default models at 0.6s, Cohere at 1.0s per SettingsStore.swift).

Because it was @Published, every write fired Combine's synthesized objectWillChange.send(). AppServices.setupASRForwarding pipes ASRService.objectWillChange into AppServices.objectWillChange, and ContentView observes AppServices via @EnvironmentObject. SwiftUI's legacy ObservableObject protocol has no per-property dependency tracking, so every tick invalidated ContentView and forced its 3,061-line body to re-evaluate, even though ContentView never reads partialTranscription.

MenuBarManager is the only consumer of this value (it forwards to the notch overlay when streaming preview is enabled). Moving the hot path off objectWillChange eliminates the invalidation storm without touching any legitimate cold-signal forwarding (isRunning, isAsrReady, micStatus, errorTitle, etc. stay @Published and ContentView continues to react to them correctly).

Approach

1. Replace @Published var partialTranscription: String = "" with private let partialTranscriptionSubject = CurrentValueSubject<String, Never>("").
2. Expose var partialTranscription: String { subject.value } for synchronous reads and var partialTranscriptionPublisher: AnyPublisher<String, Never> for subscription.
3. Update the three writers in ASRService to call partialTranscriptionSubject.send(...) instead of assigning the property.
4. Update MenuBarManager.configure(asrService:) to subscribe via asrService.partialTranscriptionPublisher instead of asrService.$partialTranscription.

Non-goals: no changes to any other @Published property, no migration to @Observable, no changes to AppServices forwarding or ContentView. This is the minimal surgical fix to the documented hot path.

Validation

• SwiftLint strict: 0 violations on both edited files.
• xcodebuild build: clean with respect to this change. (The swift-sdk MCP transport has two pre-existing data-race errors on upstream/main that remain; confirmed identical on the unmodified branch.)
• Manual verification of the value flow: partialTranscriptionSubject.send(x) updates the getter and the published publisher on the next subscription tick. MenuBarManager's .receive(on: DispatchQueue.main).sink { ... } still delivers on main as before.

Risks / follow-ups

• Any future code that subscribes to $partialTranscription via the Combine projected value will break and must use partialTranscriptionPublisher instead. Currently there are no other consumers.
• Broader cleanup opportunities remain in the AppServices forwarding design (blanket objectWillChange forwarding for AudioHardwareObserver still exists, and migrating ASRService to the @Observable macro would give per-property tracking across all fields). Those are out of scope for this PR.

[Copilot]

Pull request overview

Reduces SwiftUI invalidation during streaming ASR by taking partialTranscription off ObservableObject.objectWillChange and exposing it via a dedicated Combine publisher for the single consumer (MenuBarManager).

Changes:

• Replaced @Published partialTranscription with a private CurrentValueSubject plus a synchronous getter and AnyPublisher accessor.
• Updated all internal writers to send(...) through the subject.
• Updated MenuBarManager to subscribe via partialTranscriptionPublisher instead of $partialTranscription.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
Sources/Fluid/Services/ASRService.swift	Moves hot-path partial transcription updates to a CurrentValueSubject to avoid objectWillChange storms.
Sources/Fluid/Services/MenuBarManager.swift	Switches subscription from $partialTranscription to the new partialTranscriptionPublisher.

Comments suppressed due to low confidence (1)

Sources/Fluid/Services/MenuBarManager.swift:90

• The sink closure doesn’t reference self, so [weak self] plus guard self != nil is redundant. Consider removing the weak capture/guard (or, if you intend to use self later, unwrap with guard let self = self else { return } and actually use self).

            .sink { [weak self] newText in
                guard self != nil else { return }
                // CRITICAL FIX: Check if streaming preview is enabled before updating notch

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