Add machine comfort settings and tighten the document schemas

Machine-based exercises now carry ordered name/value comfort settings
(seat height, back-rest position, ...) on both ExerciseDocument and, as a
plan-time snapshot, WorkoutLogDocument — the optional array doubles as the
machine flag. Editable from the exercise editor's new Machine section and
from the workout row's settings sheet, whose mid-workout edits write back
to the originating split (workout saved first, so seed clone-on-edit
repointing can't clobber the log edit).

Schema tightening rides the same rev: splits bump to v2 (weight-reminder
fields and the unused exercise category removed), workouts to v3 (the
derived `completed` flag removed; status is the single source). Starter
seeds regenerated at v2 with unchanged ULIDs; SwiftData cache schema
bumped to rebuild. SCHEMA.md documents the shapes.

Claude-Session: https://claude.ai/code/session_01LEoff8bXGBS83tK1c55Mf7

Workouts/Views/WorkoutLogs/PlanEditView.swift

@@ -160,10 +160,12 @@ struct PlanEditView: View {
         let exerciseName = doc.logs[i].exerciseName
         let workoutDoc = doc
 
-        // 2) Mirror the plan onto the matching exercise in the split template.
+        // 2) Mirror the plan onto the matching exercise in the split template,
+        // following the seed clone-on-edit redirect so a prior fork doesn't
+        // silently break the mirror.
         var splitDoc: SplitDocument?
         if let splitID = doc.splitID,
-           let split = CacheMapper.fetchSplit(id: splitID, in: modelContext) {
+           let split = CacheMapper.fetchSplit(id: sync.currentSplitID(for: splitID), in: modelContext) {
             var sDoc = SplitDocument(from: split)
             if let ei = sDoc.exercises.firstIndex(where: { $0.name == exerciseName }) {
                 sDoc.exercises[ei].sets = sets