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.

Shared/Model/Documents.swift

@@ -29,6 +29,12 @@ struct SplitDocument: Codable, Sendable, Equatable, Identifiable {
     /// older app dropping it on rewrite (reverting to the default) is preferable to
     /// quarantining the user's whole routine.
     var activityType: Int?
+    /// Per-split rest length (seconds) and flow-mode toggle. Both optional and
+    /// deliberately NOT schema-bumped, same rationale as `activityType`: an older
+    /// app dropping them on rewrite reverts to the global default rest / no-flow,
+    /// which is preferable to quarantining the user's whole routine.
+    var restSeconds: Int? = nil
+    var autoAdvance: Bool? = nil
 
     // Bumped 1→2 when the weight-reminder fields (`weightLastUpdated`,
     // `weightReminderWeeks`) and `category` were removed and `machineSettings` was
@@ -103,6 +109,13 @@ struct WorkoutDocument: Codable, Sendable, Equatable, Identifiable {
     /// period. See `WorkoutMergePlanner`.
     var deletedLogIDs: [String: Date]? = nil
 
+    /// Snapshot of the split's rest length / flow flag at plan time (a running
+    /// workout has no live link to its split, same reason sets/reps/weight are
+    /// snapshotted per log). Optional and not schema-bumped, same rationale as
+    /// `SplitDocument.restSeconds`/`autoAdvance`.
+    var restSeconds: Int? = nil
+    var autoAdvance: Bool? = nil
+
     // Bumped 1→2 when `metrics` was added: the captured HR/calorie data is
     // irreplaceable, so the forward-gate must quarantine these files on older apps
     // rather than let them strip `metrics` on rewrite.