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.

Exercise Library/Reverse Crunch/motion.json

@@ -0,0 +1,22 @@
+{
+  "name": "Reverse Crunch",
+  "primary": 2,
+  "working": ["spine"],
+  "hide": ["arm_r"],
+  "frames": [
+    {
+      "hold": 0.5, "tween": 0.7,
+      "root": [172, 148],
+      "spine": [180, 180], "neck": 168, "gaze": 90,
+      "arm_l": [-2, -2],
+      "leg_r": [100, -20], "leg_l": [100, -20]
+    },
+    {
+      "hold": 0.8, "tween": 0.9,
+      "root": [166, 136],
+      "spine": [195, 175], "neck": 182, "gaze": 90,
+      "arm_l": [-2, -2],
+      "leg_r": [115, -10], "leg_l": [115, -10]
+    }
+  ]
+}