Add spoken exercise instructions (on-device TTS)

Speak exercise setup and form cues aloud with AVSpeechSynthesizer:

- Library detail: a speaker toolbar button reads the full reference aloud.
- Active workout: an opt-in "Speak Exercise Cues" setting speaks a brief
  cue (Setup/Execution/Cues) when an exercise starts, hands-free.
- Settings › Voice: pick the voice (auto-prefers an installed enhanced/
  premium English voice), a premium-download nudge shown only while on a
  basic voice, and a Speed/Pitch/Volume sheet with Reset to Defaults.

On-device and offline; ducks other audio rather than stopping it. iPhone
only for now. Shared SpeechSettings is the single source of truth for the
voice/prosody, read fresh per utterance so changes preview live.

Claude-Session: https://claude.ai/code/session_01BQcEWmAPA78338QuEwRkAh

Workouts/Views/Exercises/ExerciseLibraryView.swift

@@ -28,6 +28,7 @@ struct ExerciseLibraryView: View {
 /// progression chart; the looping form-guide figure holds the bottom.
 struct ExerciseLibraryDetailView: View {
     @Environment(SyncEngine.self) private var sync
+    @Environment(AppServices.self) private var services
 
     let exerciseName: String
 
@@ -143,6 +144,25 @@ struct ExerciseLibraryDetailView: View {
         }
         .navigationTitle(exerciseName)
         .navigationBarTitleDisplayMode(.inline)
+        .toolbar {
+            if let info {
+                ToolbarItem(placement: .primaryAction) {
+                    let announcer = services.speechAnnouncer
+                    Button {
+                        if announcer.isSpeaking {
+                            announcer.stop()
+                        } else {
+                            announcer.speak(info.spokenScript(name: exerciseName, brief: false))
+                        }
+                    } label: {
+                        Image(systemName: announcer.isSpeaking ? "stop.fill" : "speaker.wave.2.fill")
+                    }
+                    .accessibilityLabel(announcer.isSpeaking ? "Stop reading" : "Read instructions aloud")
+                }
+            }
+        }
+        // Leaving the screen shouldn't keep narrating from a page you can no longer see.
+        .onDisappear { services.speechAnnouncer.stop() }
         .sheet(item: $settingsEdit) { route in
             MachineSettingsSheet(exerciseName: exerciseName, initialSettings: route.initial) { updated in
                 let targets = route.targetSplitIDs