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/Utils/Date+Extensions.swift

@@ -18,8 +18,16 @@ extension Date {
     private static let weekdayAbbrev: DateFormatter = {
         let f = DateFormatter(); f.dateFormat = "EEE"; return f
     }()
+    private static let relativeDateTime: DateFormatter = {
+        let f = DateFormatter()
+        f.dateStyle = .medium; f.timeStyle = .short
+        f.doesRelativeDateFormatting = true
+        return f
+    }()
 
     func formattedDate() -> String { Self.shortDateTime.string(from: self) }
+    /// "Today at 10:44 AM" / "Yesterday at 9:12 AM" / "Jul 3, 2026 at 8:00 AM".
+    func formattedRelativeDateTime() -> String { Self.relativeDateTime.string(from: self) }
     func formattedTime() -> String { Self.timeOnly.string(from: self) }
     func formatDate() -> String { Self.mediumDate.string(from: self) }