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/Plank/motion.json

@@ -0,0 +1,21 @@
+{
+  "name": "Plank",
+  "primary": 2,
+  "working": ["spine"],
+  "frames": [
+    {
+      "hold": 0.4, "tween": 0.7,
+      "root": [172, 114],
+      "spine": [185, 185], "neck": 185, "gaze": 210,
+      "arm_r": [-90, 180], "arm_l": [-90, 180],
+      "leg_r": [-55, 0], "leg_l": [-55, 0]
+    },
+    {
+      "hold": 1.8, "tween": 0.7,
+      "root": [168, 137],
+      "spine": [170, 170], "neck": 170, "gaze": 210,
+      "arm_r": [-90, 180], "arm_l": [-90, 180],
+      "leg_r": [-10, -10], "leg_l": [-10, -10]
+    }
+  ]
+}