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/Dead Bug/motion.json

@@ -0,0 +1,21 @@
+{
+  "name": "Dead Bug",
+  "primary": 2,
+  "working": ["arm_r", "leg_l"],
+  "frames": [
+    {
+      "hold": 0.5, "tween": 0.8,
+      "root": [185, 148],
+      "spine": [180, 180], "neck": 168, "gaze": 90,
+      "arm_r": [90, 90], "arm_l": [90, 90],
+      "leg_r": [64, -10], "leg_l": [64, -10]
+    },
+    {
+      "hold": 0.9, "tween": 0.8,
+      "root": [185, 148],
+      "spine": [180, 180], "neck": 168, "gaze": 90,
+      "arm_r": [135, 135], "arm_l": [90, 90],
+      "leg_r": [64, -10], "leg_l": [16, 16]
+    }
+  ]
+}