Give cross-body bars a true 3D axis and fix the goblet squat hold

The orbit was always a real camera orbit — figure and props share one
rigid rotation — but a bar's screen-space angle authored the wrong 3D
rod: the default horizontal encoded a rod along the body axis, so
barbells hovered fixed on screen and vanished at the head-on view
where they should span widest. Line props now take "axis": "z" (both
renderers in lockstep, fixture-pinned): the world left-right direction
projects through the camera pitch like the floor quad — end-on plates
in profile, full span face-on, swinging with the hands in between.
Applied to the ten cross-body bars; vertical handles were already
orbit-invariant.

Goblet Squat's hand pins sat so close to the shoulders that the
two-bone IK was degenerate, flipping between a chicken-wing and an
elbow-behind solve; re-pinned level with the shoulders so the elbows
tuck straight down through the whole rep.

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

Exercise Library/SYSTEM.md

@@ -179,12 +179,21 @@ authored yaw nothing moves, so the authored look is exact.
   (`hand_r`, `foot_l`, …) plus the mid joints (`elbow_r`, `knee_l`, …), so a
   machine pad can ride a knee (`["knee_r", "knee_l"]`) or span a shin
   (`["knee_r", "foot_r"]`).
-  `bar` lies at a fixed authored-view `angle` (default 0 = horizontal — in
-  side view a two-handed bar is drawn horizontal by convention); `dumbbell`
-  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).
+  `bar` lies at a fixed authored-view `angle` (default 0 = horizontal);
+  `dumbbell` 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 cross-body rod — a barbell, pull-up bar, or any grip that really runs
+  left-right *through* both hands — should instead set `"axis": "z"`
+  (superseding `angle`): its world-space direction projects through the
+  camera elevation like the floor quad, so in a profile view it reads
+  end-on (plates nearly concentric, seen from slightly above), opens to
+  its full span face-on, and swings with the figure in between. A fixed
+  screen-space `angle` can't do this — it keeps the rod glued to the
+  authored direction while the body turns under it. Vertical handles
+  (`angle: 90`) don't need it: a world-vertical rod is unchanged by a yaw
+  orbit.
 - **`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