Preview the next exercise and count into each set during rests

The run flow's between-exercise rest now previews the next exercise — its
figure and a large "Coming up" name — and the hands-free narration gains a
per-second count-in ("<Exercise>, in 1, 2, 3, GO!") spoken before every set,
plus a "Coming up" announcement as a between-exercise rest begins. A new
Settings > Narration picker chooses the count-in cues, the setup/form read,
or both.

Also fixes spoken cues going silent after the first exercise in a flow split:
the stop-on-teardown moved from the per-exercise view (rebuilt on every
hand-off) to the run host, which stays mounted for the whole run. The audio
session now holds its duck across the per-second count so background music
doesn't pulse between words.

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

CHANGELOG.md

@@ -1,5 +1,13 @@
 **July 2026**
 
+While you rest between exercises, the workout screen now previews the next exercise's figure and name so you can see what's coming up.
+
+Spoken cues can now announce the next exercise during a rest and count you into every set with "in 1, 2, 3, GO!".
+
+A new Narration setting picks whether spoken cues give the coming-up countdown, the full setup and form read, or both.
+
+Spoken cues no longer stop after the first exercise in an auto-advancing workout.
+
 The watch now starts recording on its own whenever a workout is active, so a missed hand-off from the iPhone no longer leaves a session without heart rate or Health credit.
 
 A workout recording now survives the watch app crashing or the watch restarting, reconnecting to the in-progress session instead of losing it.