Add per-split rest length and hands-free auto-advance flow

Two per-split settings, with the global Settings values as defaults:

- restSeconds: Int? — per-split rest, used between sets and (in flow) between
  exercises; nil falls back to the global default.
- autoAdvance: Bool? — flow mode: finishing an exercise rests, then opens the
  next one hands-free, all the way through the split.

Both are optional, snapshotted onto WorkoutDocument at the start sites (no live
split link), and not schema-bumped — same degradation pattern as activityType.

A thin RunFlowView wrapper (iOS + watch) owns the on-screen log and swaps it via
.id(currentLogID) on hand-off, so the per-exercise ExerciseProgressView stays
per-logID and untouched; the between-exercise rest reuses the existing .rest
countdown as the terminal page. The mirror reuses the per-logID live channel:
the wrapper suppresses the boundary .ended teardown so it follows across
exercises, and ContentView re-keys the cover on frame.logID — no sync-bridge
changes.

Morning Wake-Up ships as a flowing 45s-work / 15s-rest routine.

New Rest & Pacing section in the split editor exposes both controls.

README.md

@@ -7,11 +7,14 @@ your own iCloud Drive.
 ## Key Features
 
 - **Workout splits** — organize exercises into reusable routines with custom
-  colors, icons, and an activity type (strength, HIIT, cardio, …). Built-in
-  starter splits (Upper Body / Core / Lower Body
-  machine routines, plus an equipment-free **Bodyweight Core** circuit with its
-  own warm-up) appear automatically on first launch; editing one makes it your
-  own copy, and deleted starters can be restored from Settings.
+  colors, icons, and an activity type (strength, HIIT, cardio, …). Each split can
+  set its own rest time (overriding the app-wide default) and turn on **Auto-Advance**
+  to flow hands-free from one exercise to the next, resting automatically between them
+  (the **Morning Wake-Up** starter split ships this way). Built-in starter splits
+  (Upper Body / Core / Lower Body machine routines, plus an equipment-free
+  **Bodyweight Core** circuit with its own warm-up) appear automatically on first
+  launch; editing one makes it your own copy, and deleted starters can be restored
+  from Settings.
 - **Exercise library** — a bundled library of 47 exercises to populate your
   splits, authored in `Exercise Library/` with per-exercise setup, cues,
   mistakes, progressions, and an anatomically-rigged 3D stick-figure visual