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

Workouts/ExerciseFigure/ExerciseMotion.swift

@@ -178,6 +178,10 @@ struct MotionProp: Codable {
     /// Fixed world angle (degrees, y-up). Default: bars are horizontal;
     /// dumbbells/pads sit perpendicular to the lower bone.
     let angle: Double?
+    /// True 3D axis, superseding `angle`: `"z"` marks a cross-body rod
+    /// (barbell, pull-up bar) whose projection foreshortens naturally —
+    /// end-on in a profile view, full span face-on.
+    let axis: String?
     let halfLen: Double?
     let w: Double?
     /// End-disc radius (dumbbell plates default 4.5; bars none).