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/Bird Dog.motion.json

@@ -0,0 +1,23 @@
+{
+  "name": "Bird Dog",
+  "primary": 2,
+  "working": ["arm_r", "leg_l"],
+  "frames": [
+    {
+      "hold": 0.5, "tween": 0.8,
+      "root": [190, 106],
+      "spine": [171, 171], "neck": 187, "gaze": 205,
+      "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": 1.4, "tween": 0.8,
+      "root": [190, 106],
+      "spine": [171, 171], "neck": 187, "gaze": 195,
+      "arm_r": [161, 161], "arm_l": [-90, -90],
+      "leg_r": [-83, 0], "leg_l": [15, 15],
+      "pins": {"hand_l": [111, 154]}
+    }
+  ]
+}