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/Reverse Crunch/info.md

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+# Reverse Crunch
+
+Curl the pelvis toward the ribs — lower-ab flexion without neck strain.
+
+- **Category:** Main circuit
+- **Type:** Spinal flexion (lower)
+- **Targets:** Lower rectus abdominis, deep core
+- **Prescription:** 3 × 12–15 reps, controlled negative
+
+## Setup
+
+Lie on your back, knees bent toward the chest, shins roughly parallel to the
+floor, arms along your sides pressing lightly into the floor.
+
+## Execution
+
+1. Exhale and curl the pelvis up and toward the ribs, lifting the hips off the
+   floor — knees travel toward the chin.
+2. Pause briefly at the top.
+3. Lower the hips back down slowly, one vertebra at a time.
+
+## Cues
+
+- Curl, don't kick: the knees drifting up should come from the pelvis tilting,
+  not from leg swing.
+- Controlled negative — resist gravity all the way down.
+- Keep the head and shoulders relaxed on the floor.
+
+## Common Mistakes
+
+- Swinging the legs and lifting with momentum.
+- Pushing hard through the arms instead of the abs.
+- Letting the hips drop back down in one piece.
+
+## Progression
+
+Feet-elevated pelvic tilts → reverse crunch → slow 3-count negatives →
+incline (head-up) reverse crunch → dragonflag negatives