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/Hollow Body Hold/info.md

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+# Hollow Body Hold
+
+The gold standard for deep core tension — a shallow, rigid banana shape.
+
+- **Category:** Main circuit
+- **Type:** Anti-extension (isometric hold)
+- **Targets:** Rectus abdominis, deep core, hip flexors
+- **Prescription:** 3 × 20–40 s holds
+
+## Setup
+
+Lie on your back, arms extended overhead beside the ears, legs straight and
+together.
+
+## Execution
+
+1. Press the lower back *into* the floor and hold it there — this never releases.
+2. Lift shoulder blades and legs a few inches off the floor into a shallow
+   banana shape.
+3. Reach long through fingers and toes and hold.
+
+## Cues
+
+- The lower back stays welded to the floor; if it lifts, shorten the shape.
+- Chin gently tucked — look at your thighs, not the ceiling.
+- Lower = harder. Height is not the goal, tension is.
+
+## Common Mistakes
+
+- Lower back arching off the floor (the hold has failed at that point).
+- Folding at the hips into a V-sit.
+- Neck cranked forward doing all the "work".
+
+## Progression
+
+Tuck hold (knees in) → one leg extended → full extension, arms overhead →
+hollow rocks