Add the exercise reference library, animated exercise figures, and exercise categories

Exercise Library/ holds per-exercise reference docs (setup, cues,
mistakes, progressions) with SVG visuals and a Python-rendered motion
pipeline; Workouts/ExerciseFigure renders the bundled *.motion.json
rigs as animated stick figures on the exercise screen. Exercises gain
a warm-up/main-circuit category, timed exercises display hold time via
planSummary, and a completed exercise reopens to a check screen instead
of its timers.

Shared/Model/Documents.swift

@@ -46,6 +46,10 @@ struct ExerciseDocument: Codable, Sendable, Equatable, Identifiable {
     var durationSeconds: Int       // total seconds (0 when not a timed exercise)
     var weightLastUpdated: Date?
     var weightReminderWeeks: Int
+    /// Raw `ExerciseCategory`; nil → main circuit. Like `activityType`, deliberately
+    /// NOT schema-bumped: it only affects how the split screen groups its list, so an
+    /// older app dropping it on rewrite beats quarantining the routine.
+    var category: Int?
 }
 
 // MARK: - Workout