Give machine props world-space 3D form that rotates with the camera

Scene shapes, cable anchors, bar angles, pad perpendiculars, and roller
offsets all resolve in the authored view exactly as before, then rotate
about the world-vertical axis through the root anchor - the same
resolve-then-rotate pattern as the figure's pins and the mat - so at the
authored yaw every exercise renders bit-identically to today, and under
an orbiting camera the equipment turns with the figure while staying
welded to its hands and feet. Scene lines gain an optional depth plane
(z) and slab extrusion (depth) so seats, backrests, and platforms keep
form edge-on; the rect shape is retired (re-authored as slab lines).
All 14 machines' props re-authored with depths and verified at eight
orbit angles. The fixture snapshots move into the pipeline as
render.py --fixtures and now cover orbit-presentation samples with
resolved prop primitives for a spread of prop flavors; the in-app
renderer resolves props in MotionSolver (lockstep with resolve_props)
and the view just draws primitives.

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

Exercise Library/Leg Curl/frames/frame-1.svg

@@ -1,12 +1,12 @@
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