Anchor auto-advanced phases at the boundary, not at tick time

The rest/timed-work countdown deadline is shared by both devices, but the
page flip crossing it is a local ticker event — and stamping the *next*
phase's anchor at Date() when that event finally ran baked a sleeping
watch's lateness (throttled wrist-down ticker) into its next count-up,
leaving the two devices permanently offset with nothing on the wire to
correct.

Auto-advances now chain the anchor instead: the finished phase's computed
end (passed out of CountdownPhaseView) becomes the next page's PageAnchor,
with the next window derived from it via liveSnapshot(for:at:). A device
arbitrarily late to a boundary shows exactly what the on-time device shows,
and a stack of missed boundaries fast-forwards itself — each chained page
lands already-elapsed and advances on its own next tick, skipping the
start/stop haptics for boundaries that passed while asleep (only a
just-crossed boundary buzzes).

The remoteAnchor* fields are generalized into one PageAnchor (remote frames
and chained auto-advances are the same concept: a page whose timer counts
from a known instant); the phone's Live Activity emit honors it unchanged.

CHANGELOG.md

@@ -1,5 +1,7 @@
 **July 2026**
 
+Watch and iPhone set timers no longer drift apart when the watch sleeps through the end of a rest.
+
 Swiping through sets on the iPhone now reliably moves the watch along too, instead of the two drifting apart mid-workout.
 
 On Apple Watch, the workout screen now gives its whole display to the set and rest timer, without the form-guide figure shown on iPhone.