Draw both arms on the supine motions and retire the hide list

Hollow Body Hold, Leg Raises, and Reverse Crunch hid the far arm
because it was never authored - a leftover from the planar rig. Both
arms are now posed, so the far arm reads as the standard light member
behind the near one, the same visual language every other exercise
uses, and the figures stay truthful from any viewpoint. With no
remaining users, the hide mechanism is deleted from both renderers,
the motion schema, and the docs.

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

Exercise Library/SYSTEM.md

@@ -45,7 +45,6 @@ never touches a motion script** — proportions are the skeleton's problem.
   "primary": 2,                     // 1-based frame used for visual.svg
   "camera": {"yaw": 0},             // 0 = side view (default), 90 = face-on
   "working": ["arm_l", "leg_r"],    // parts drawn in the accent color
-  "hide": [],                       // limbs occluded by convention, not drawn
   "frames": [
     {
       "hold": 0.5, "tween": 0.8,    // seconds held / animating to the next frame
@@ -115,8 +114,9 @@ never touches a motion script** — proportions are the skeleton's problem.
   shoulder girdle and pelvis drawn as bars across the attach points (near-full
   width face-on, a shoulder/hip nub in profile); teal when the trunk is the
   working part.
-- Canvas 320×180, ground line at y = 152. Limbs listed in `hide` are
-  occluded by convention in this view and not drawn.
+- Canvas 320×180, ground line at y = 152. Every limb is always drawn — a
+  supine figure's far arm reads as the light far member, never a hidden one,
+  so any viewpoint (and the orbit) stays truthful.
 
 ## The props layer
 
@@ -172,8 +172,7 @@ authored yaw nothing moves, so the authored look is exact.
   and `pad` default to perpendicular to the lower bone (forearm/shin), or
   take an explicit `angle`. Under orbit the segment rotates with the scene
   and foreshortens naturally. `plateR` puts filled discs on both ends
-  (dumbbells default to 4.5). A prop whose limb is hidden that frame simply
-  isn't drawn.
+  (dumbbells default to 4.5).
 - **`roller`** — a machine roller pad seen end-on: a filled disc riding the
   limb's lower bone near the joint in `at`, on the `side` (+1/−1) of the bone
   it presses — a leg extension's instep roller (`side: 1`), a leg curl's