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.

WorkoutsTests/SplitDocumentCodableTests.swift

@@ -21,7 +21,8 @@ struct SplitDocumentCodableTests {
             loadType: 0,
             durationSeconds: 0,
             weightLastUpdated: Self.stableTimestamp,
-            weightReminderWeeks: 4
+            weightReminderWeeks: 4,
+            category: ExerciseCategory.warmup.rawValue
         )
         let original = SplitDocument(
             schemaVersion: SplitDocument.currentSchemaVersion,
@@ -42,4 +43,34 @@ struct SplitDocumentCodableTests {
         #expect(decoded == original)
         #expect(decoded.relativePath == "Splits/\(original.id).json")
     }
+
+    /// A pre-category exercise (no `category` key) must still decode — the field is
+    /// optional and deliberately not schema-bumped, defaulting to the main circuit.
+    @Test func decodesLegacyExerciseWithoutCategory() throws {
+        let json = """
+        {
+            "schemaVersion": 1,
+            "id": "01HZZZZZZZZZZZZZZZZZZZZZZZ",
+            "name": "Push Day",
+            "color": "blue",
+            "systemImage": "dumbbell.fill",
+            "order": 0,
+            "createdAt": "2023-11-14T22:13:20Z",
+            "updatedAt": "2023-11-14T22:13:20Z",
+            "exercises": [{
+                "id": "01HZZZZZZZZZZZZZZZZZZZZZZY",
+                "name": "Bench Press",
+                "order": 0,
+                "sets": 4,
+                "reps": 10,
+                "weight": 135,
+                "loadType": 1,
+                "durationSeconds": 0,
+                "weightReminderWeeks": 4
+            }]
+        }
+        """
+        let decoded = try DocumentCoder.decode(SplitDocument.self, from: Data(json.utf8))
+        #expect(decoded.exercises.first?.category == nil)
+    }
 }