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

WorkoutsTests/ExerciseMotionTests.swift

@@ -80,8 +80,8 @@ struct ExerciseMotionTests {
         #expect(hypot(releasedHand.x - releasedHand0.x, releasedHand.y - releasedHand0.y) > 1)
     }
 
-    /// Prop-free bodyweight motions slowly orbit the camera while looping; motions with
-    /// equipment (view-locked scene shapes) or a hide list keep their authored view.
+    /// Prop-free bodyweight motions slowly orbit the camera while looping; motions
+    /// with equipment keep their authored view (until machines orbit too).
     @Test func orbitAppliesOnlyToPropFreeMotions() throws {
         let birdDog = try #require(FigureAnimation(exerciseName: "Bird Dog"))
         #expect(birdDog.orbits)