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/info.md

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+# Plank
+
+The baseline anti-extension hold — a rigid line from head to heels.
+
+- **Category:** Main circuit
+- **Type:** Anti-extension (isometric hold)
+- **Targets:** Rectus abdominis, deep core, glutes, shoulders
+- **Prescription:** 3 × 30–60 s holds
+
+## Setup
+
+Forearms on the floor, elbows under shoulders, feet together behind you. One
+straight line from the back of the head to the heels.
+
+## Execution
+
+1. Squeeze glutes and quads hard, tuck ribs down toward the pelvis.
+2. Press the floor away through the forearms — no sagging between the shoulders.
+3. Breathe shallowly behind the brace and hold.
+
+## Cues
+
+- Glutes and quads *tight* — a plank fails at the hips first.
+- Ribs down: think "short front, long back".
+- If the hips sag or the lower back takes over, the set is done.
+
+## Common Mistakes
+
+- Hips sagging (lower back extension — the exact thing to resist).
+- Hips piked high to make the hold easier.
+- Holding the breath entirely.
+
+## Progression
+
+Knee plank → full plank → feet elevated → RKC plank (maximum-tension, shorter
+holds) → weighted plank