Merge watch workout pushes per log instead of whole-document

ingestFromWatch arbitrated by whole-document updatedAt, so concurrent edits
to the same workout on both devices (phone edits exercise A while the watch
completes exercise B) lost one side wholesale — the newer snapshot replaced
the other (H1).

Reconcile per log instead. WorkoutMergePlanner (pure, deterministic) unions
logs by id, resolves each by newest per-log updatedAt, and applies
phone-authored deletion tombstones so an absent log is never ambiguous
between "deleted on the phone" and "just added on the watch". Edits to
different exercises now commute — delivery order and offline gaps stop
mattering. A stale/duplicate push merges back to exactly the cached doc, so
ingest re-pushes authoritative state rather than writing.

The per-log updatedAt scaffolding shipped (unused) in schema v4; it's now
stamped by transition(to:) on status flips and a new touch() at the
content-only edit sites (order, notes, machine settings, adjusted entries,
new logs) on both phone and watch. deletedLogIDs is new: additive on the
wire and cache, phone-authored (deleteLog), pruned after a 30-day grace.
Because an older build rewriting a file would strip the tombstones and
resurrect a deleted exercise, WorkoutDocument schema bumps 4->5 (forward
gate quarantines old builds) and the cache bumps 5->6.

recomputeStatusFromLogs takes an injectable now: so the merge recomputes
status/end deterministically. WorkoutMergePlannerTests pins the decision
table (commute, per-log newer-wins, legacy-nil, watch-add, tombstone
honored/resurrect/union/prune, status recompute, no-op re-push);
WorkoutDocumentMapperTests gains the deletedLogIDs round-trip.

Workouts/Views/WorkoutLogs/ExerciseProgressView.swift

@@ -605,6 +605,7 @@ struct ExerciseProgressView: View {
         guard let i = doc.logs.firstIndex(where: { $0.id == logID }),
               let last = doc.logs[i].setEntries?.indices.last else { return }
         doc.logs[i].setEntries?[last] = entry
+        doc.logs[i].touch()
         doc.updatedAt = Date()
         onChange()
     }