Give the figure real girdles, fix orbit pinning, add roller pads

Shoulder and pelvis widths grow to human-like proportions per profile
(shoulders wider than hips for neutral/male, reversed for female) and
are now drawn — bars across the attach points that read near-full-width
face-on and as a shoulder/hip nub in profile, so limbs visibly hang
from a torso instead of a point. Orbiting no longer re-solves IK pins
in the rotated view (pins are canvas targets in the authored camera):
the pose resolves first and the posed body rotates, which fixes hands
sticking to stale screen points mid-orbit (Cat-Cow, Bird Dog, Plank).
Leg Extension and Leg Curl swap their ankle bars for a machine roller
disc — a new `roller` prop riding the shin's press side. Fixtures
regenerated; both renderers updated in lockstep.

Claude-Session: https://claude.ai/code/session_01LEoff8bXGBS83tK1c55Mf7

Exercise Library/SYSTEM.md

@@ -103,8 +103,10 @@ never touches a motion script** — proportions are the skeleton's problem.
   stays distinguishable. The nudge scales with how side-on the view is and
   vanishes face-on, where the skeleton's real shoulder/pelvis widths take
   over — one continuous rule across the whole orbit.
-- **Spine** — rendered as a smooth curve through pelvis → mid → neck; teal
-  when the trunk is the working part.
+- **Spine** — rendered as a smooth curve through pelvis → mid → neck, with the
+  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.
 
@@ -150,6 +152,11 @@ scale drawings of the machine.
   default to perpendicular to the lower bone (forearm/shin), or take an
   explicit `angle`. `plateR` puts filled discs on both ends (dumbbells default
   to 4.5). A prop whose limb is hidden that frame simply isn't drawn.
+- **`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
+  heel roller (`side: -1`). `r` is the radius, `back` slides it along the
+  bone away from the joint.
 
 ## Rendering