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.

Workouts/Resources/ExerciseMotions/Cat-Cow.motion.json

@@ -0,0 +1,23 @@
+{
+  "name": "Cat-Cow",
+  "primary": 1,
+  "working": ["spine"],
+  "frames": [
+    {
+      "hold": 0.8, "tween": 0.9,
+      "root": [190, 106],
+      "spine": [188, 155], "neck": 185, "gaze": 164,
+      "arm_r": [-90, -90], "arm_l": [-90, -90],
+      "leg_r": [-83, 0], "leg_l": [-83, 0],
+      "pins": {"hand_r": [105, 152], "hand_l": [111, 154]}
+    },
+    {
+      "hold": 0.8, "tween": 0.9,
+      "root": [190, 106],
+      "spine": [155, 192], "neck": 215, "gaze": 225,
+      "arm_r": [-90, -90], "arm_l": [-90, -90],
+      "leg_r": [-83, 0], "leg_l": [-83, 0],
+      "pins": {"hand_r": [105, 152], "hand_l": [111, 154]}
+    }
+  ]
+}