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/AppServices.swift

@@ -19,6 +19,10 @@ final class AppServices {
     let workoutLauncher = WorkoutLauncher()
     let workoutHealthDeleter = WorkoutHealthDeleter()
 
+    /// Speaks exercise instructions aloud (library Speak button + hands-free workout cues).
+    /// Shared so a new exercise's cue cancels whatever the last screen was still saying.
+    let speechAnnouncer = SpeechAnnouncer()
+
     /// Owns the active-workout Live Activity (lock screen + Dynamic Island). Driven by the run
     /// flow's `LiveProgress` frames, so locking the phone mid-set doesn't lose the timer.
     let liveActivity = WorkoutLiveActivityController()