Target handle (top of card): Enter/Space extracts this node from
its parent, moving it to the root level.
Source handle (bottom of card): Enter/Space nests the currently
selected node as a child of this node (requires another node to be
selected first).
Both handles gain tabIndex=0, role="button", a descriptive aria-label,
and a blue focus ring so keyboard-only users can navigate the
workspace hierarchy without a mouse. Uses the existing nestNode store
action — no new API surface needed.
Updates the canvas audit doc to mark the LOW edge-anchor item done.
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>