Add presentation-only camera.zoom so standing motions fit the canvas

The rig is ~211 canvas units tall standing but the canvas has 152 above
the ground line - every motion to date is seated or on the floor. A
per-motion "camera": {"zoom": ...} now scales the drawn output (figure,
props, mat, stroke widths) about the ground-center anchor in both the
reference renderer and the in-app view. Pure view transform: pins, prop
coordinates, and the Swift-solver fixtures stay in full-size authored
units; zoom 1 is byte-identical to before.

Claude-Session: https://claude.ai/code/session_01HJDQQDA9QdP8zByg43H5v3

Exercise Library/SYSTEM.md

@@ -80,6 +80,16 @@ never touches a motion script** — proportions are the skeleton's problem.
   camera also orbits while the motion loops (`--orbit`; the in-app renderer
   slowly orbits **every** exercise, machines included — props have
   world-space 3D form and turn with the figure).
+- **Zoom** — the skeleton is ~211 canvas units tall standing but the canvas
+  has only 152 above the ground line, so standing motions author full-size
+  anatomy (coordinates may run past the canvas top) and set
+  `"camera": {"zoom": 0.7}` to fit. Zoom is **presentation-only**: a uniform
+  scale of the drawn output (geometry, props, mat, stroke widths) about the
+  ground-center anchor `(160, 152)`, applied after all solving — pins, prop
+  coordinates, and the Swift-solver fixtures stay in full-size authored
+  units, and the ground line maps to itself so planted feet stay planted.
+  Typical values: ~0.7 standing tall, ~0.75–0.85 hanging or seated with arms
+  overhead; omit it (1) for lying, kneeling, seated, and bent-over motions.
 - **Elevation & the mat** — the default viewpoint pitches down 10°
   (`CAMERA_PITCH`; override per motion via `"camera": {"pitch": ...}`), and
   the ground is drawn as an **exercise mat**: a world-space quad sized to the