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/Common/CheckboxStatus.swift

@@ -19,8 +19,8 @@ enum CheckboxStatus {
         switch self {
         case .checked: .accentColor
         case .unchecked: .gray
-        case .intermediate: .gray
-        case .cancelled: .red
+        case .intermediate: .accentColor
+        case .cancelled: .gray
         }
     }
 
@@ -29,7 +29,7 @@ enum CheckboxStatus {
         case .checked: "checkmark.circle.fill"
         case .unchecked: "circle"
         case .intermediate: "ellipsis.circle"
-        case .cancelled: "xmark.circle"
+        case .cancelled: "wrongwaysign"
         }
     }
 }