A selected blend shows its spine. Direct Selection drags spine points
and handles, the Pen tool adds and deletes points on a selected
blend's spine (object.blend.spine.moveAnchor/addAnchor/removeAnchor).
The straight spine becomes a path on the first edit, its points pinned
to the key objects (append-only BlendSpec.key_anchors): moving a key
moves its end of the spine and the other way round; spines from older
files and Replace Spine still spread the keys evenly. Direct and Group
Selection pick blend keys on the canvas, edits redraw the blend live,
and Release leaves the spine as a path.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
