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

README.md

@@ -38,6 +38,13 @@ your own iCloud Drive.
   orbits every exercise while it moves, machine equipment (seats, bars, cables,
   roller pads) has world-space 3D form that turns with the figure, and face-on
   machines show true hip abduction and torso rotation.
+- **Spoken instructions** — tap the speaker in any library exercise to hear its
+  full reference read aloud, and turn on **Speak Exercise Cues** in Settings to
+  have the setup and form cues spoken automatically when you start an exercise, so
+  you can follow along hands-free. On-device speech that ducks — never stops — your
+  music. A **Voice** section in Settings picks the voice (preferring any enhanced or
+  premium voices you've installed) and tunes its speed, pitch, and volume, with a
+  live preview.
 - **Machine comfort settings** — machine-based exercises remember your setup
   (seat height, back-rest position, pin position, …), shown on the workout
   screen and editable mid-workout; changes save back to the split for next time.