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/Side Plank/info.md

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+# Side Plank
+
+Anti-lateral-flexion hold — resist the hips dropping toward the floor.
+
+- **Category:** Main circuit
+- **Type:** Anti-lateral flexion (isometric hold)
+- **Targets:** Obliques, quadratus lumborum, glute medius, shoulder stability
+- **Prescription:** 3 × 20–45 s per side
+
+## Setup
+
+Lie on one side, elbow directly under the shoulder, forearm across the floor.
+Feet stacked (or staggered), body in one straight line.
+
+## Execution
+
+1. Drive the hips up until shoulders, hips, and ankles form a straight line.
+2. Stack the hips vertically — don't let the top hip roll forward or back.
+3. Hold, then switch sides.
+
+## Cues
+
+- Hips *high*: most people sag an inch without noticing.
+- Push the floor away through the forearm — no collapsing into the shoulder.
+- Squeeze the bottom-side obliques and the glutes the whole hold.
+
+## Common Mistakes
+
+- Hips dropping toward the floor (the exact motion to resist).
+- Rolling the chest toward the ceiling or the floor.
+- Elbow drifting away from under the shoulder.
+
+## Progression
+
+Knees-down side plank → full side plank → top leg raised → reach-through
+(thread the top arm under the body and back up)