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/README.md

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+# Exercise Library
+
+Reference material for exercises the app knows about — one folder per exercise,
+named exactly as the exercise appears in the app.
+
+Each entry contains:
+
+- `info.md` — the details, always in this order: a one-line summary, quick facts
+  (**Category**, **Type**, **Targets**, **Prescription**), then **Setup**,
+  **Execution**, **Cues**, **Common Mistakes**, and **Progression**
+  (easier → harder — get strong by making the move harder, not by endless reps).
+- `motion.json` — the exercise scripted as key frames of joint angles on the
+  shared rig (proportions in the library-level `body.json`), with IK pins for
+  planted hands/feet and tween timings. This is the **canonical source** for
+  all visuals and for the in-app animation planned for the lower half of the
+  exercise screen.
+- Generated from it by `render.py` (never hand-edited):
+  `frames/frame-N.svg` (one per key frame), `preview.gif` (the tweened loop),
+  and `visual.svg` (the primary frame, for static contexts).
+
+The rig and the visual language — right limbs dark / left limbs light with an
+embedded R/L legend, nose tick for facing, teal on the working parts, timing
+that encodes tempo, plus figure profiles, flipping, and theming — are defined
+in `SYSTEM.md`, along with the `motion.json` schema and rendering
+instructions.
+
+The library lives at the repo root, deliberately outside the app targets'
+source folders, and is not bundled into the app yet — the in-app presentation
+format is still being worked out.