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/Bird Dog/info.md

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+# Bird Dog
+
+Anti-rotation drill — extend opposite limbs while the hips and spine refuse to
+twist.
+
+- **Category:** Warm-up and main circuit
+- **Type:** Anti-rotation
+- **Targets:** Spinal erectors, glutes, deep core, shoulder stability
+- **Prescription:** Warm-up 8 per side; main circuit 3 × 8–10 per side, slow
+
+## Setup
+
+On all fours: hands under shoulders, knees under hips, spine neutral, gaze at
+the floor.
+
+## Execution
+
+1. Brace, then reach one arm forward and the *opposite* leg back until both are
+   in line with the torso.
+2. Pause for a beat at full extension — hips stay square to the floor.
+3. Return under control and switch sides.
+
+## Cues
+
+- Imagine a cup of water balanced on the lower back — nothing spills.
+- Reach *long*, not high: heel pushes back, knuckles push forward.
+- Slow, no momentum; the pause is where the work happens.
+
+## Common Mistakes
+
+- Hips rotating open as the leg lifts.
+- Lifting the leg above hip height and arching the lower back.
+- Racing through reps.
+
+## Progression
+
+Single limb only (arm *or* leg) → opposite arm + leg → paused 3-count hold →
+elbow-to-knee touch under the body between reps → hands on an unstable surface