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/Cat-Cow/info.md

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+# Cat-Cow
+
+Gentle spinal mobility drill that wakes up the whole trunk before core work.
+
+- **Category:** Warm-up
+- **Type:** Spinal mobility
+- **Targets:** Thoracic and lumbar spine, deep core awareness
+- **Prescription:** 10 slow reps (one rep = cat + cow)
+
+## Setup
+
+On all fours: hands under shoulders, knees under hips, spine neutral, gaze at
+the floor.
+
+## Execution
+
+1. **Cat** — exhale, press the floor away, round the whole spine up toward the
+   ceiling and tuck chin and tailbone.
+2. **Cow** — inhale, let the belly drop, lift the chest and tailbone, gaze
+   slightly forward.
+3. Flow between the two, one breath per position.
+
+## Cues
+
+- Move segment by segment, not as one stiff plank.
+- Let the breath set the tempo — no rushing.
+- Reach the mid-back toward the ceiling in cat; most people only move the neck.
+
+## Common Mistakes
+
+- All the motion coming from the neck and hips, none from the mid-back.
+- Collapsing into the shoulders instead of pressing the floor away.
+
+## Progression
+
+Smaller pain-free range → full range → slower tempo with a pause at each end →
+segmental rolls (peel one vertebra at a time)