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/WorkoutLogsView.swift

@@ -220,21 +220,7 @@ struct SplitPickerSheet: View {
     private func start(with split: Split) {
         let startDate = Date()
         let logs = split.exercisesArray.enumerated().map { index, exercise in
-            WorkoutLogDocument(
-                id: ULID.make(),
-                exerciseName: exercise.name,
-                order: index,
-                sets: exercise.sets,
-                reps: exercise.reps,
-                weight: exercise.weight,
-                loadType: exercise.loadType,
-                durationSeconds: exercise.durationTotalSeconds,
-                currentStateIndex: 0,
-                completed: false,
-                status: WorkoutStatus.notStarted.rawValue,
-                notes: nil,
-                date: startDate
-            )
+            WorkoutLogDocument(planFrom: ExerciseDocument(from: exercise), order: index, date: startDate)
         }
 
         // A freshly started workout has no `end` — only completion stamps it.