Make watch HIIT progress monotonic and resume to next unfinished set

Completing a work phase advances the set count to the iPhone, and a finished
set is never un-counted — a transient paged-TabView snap to page 0 can no longer
overwrite progress with 0. Reopening an exercise now jumps to the first
unfinished set's work page (re-asserted after first layout) instead of starting
back at set 1.

Claude-Session: https://claude.ai/code/session_018gg69MaUetDNzWzBXisfMV

CHANGELOG.md

@@ -12,6 +12,10 @@ All notable changes to this project are documented here.
   on the final set.
 - Added a configurable rest-between-sets duration (iPhone Settings, default 45s),
   synced to the watch over WatchConnectivity.
+- Watch progress is now monotonic and reliably synced: completing a work phase
+  advances the set count on the iPhone and a finished set is never un-counted, and
+  reopening an exercise jumps straight to the first unfinished set (skipping
+  completed work/rest pairs) instead of snapping back to set 1.
 - Starting a workout on the iPhone now launches the Apple Watch app straight into
   the session via HealthKit (a one-time Health permission); the watch holds an
   `HKWorkoutSession` to stay active while you train and releases it when the