Port the exercise run screen's true resume to the watch

Re-entering an in-progress exercise on the watch now computes its landing
(page + wall-clock anchor) from the log's durable timestamps in init —
mid-set resumes the stopwatch from startedAt, mid-rest lands on the rest
page with the countdown continuing and auto-advancing at the true
boundary, and a spent rest anchors the next set at the rest's computed
end. Timestamps are clamped to now against peer clock skew; legacy logs
without them keep the old first-unfinished-set behavior. broadcastLive
forwards the resume anchor so a mirroring phone's timer lines up.

Claude-Session: https://claude.ai/code/session_01H8VxUX4ckjU3vRF5M4L5FV

CHANGELOG.md

@@ -1,5 +1,7 @@
 **July 2026**
 
+Reopening an exercise on the watch now also continues its timer where it left off — even mid-rest — instead of restarting from zero.
+
 A renamed exercise's headline now shows both identities on one line, like "Cardio · Warm Up".
 
 Renamed exercises in workouts recorded by older versions get their animated figure and guide back.