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

Shared/Persistence/WorkoutsModelContainer.swift

@@ -8,7 +8,9 @@ enum WorkoutsModelContainer {
     /// Bump whenever the cache schema changes — wipes and rebuilds from files.
     /// 2: added health-metric columns to `Workout` and `activityTypeRaw` to `Split`.
     /// 3: added `categoryRaw` to `Exercise`.
-    static let currentSchemaVersion = 3
+    /// 4: removed the weight-reminder columns and `categoryRaw` from `Exercise` and
+    ///    the `completed` column from `WorkoutLog`; added `machineSettings` to both.
+    static let currentSchemaVersion = 4
     private static let schemaVersionKey = "Workouts.persistenceSchemaVersion"
     private static let identityTokenKey = "Workouts.iCloudIdentityToken"