Reserve no editor height for withheld group pickers

[chubes4]

Problem

Painted source layers materialize as empty core/group blocks. In the editor, core renders its empty-group variation picker inside them, so an earlier change withheld that picker. The placeholder still reserved min-height: 2rem for it, which is height the source layer never occupies: on the frontend these layers are painted out of normal flow.

Measured in the editor canvas of a generated site, four such placeholders sit consecutively at the top of the document:

placeholder	reserved height	canvas top
1	32px	87
2	32px	119
3	32px	151
4	32px	183

That is 128px of displacement applied before any authored content, which pushes the hero and everything after it down the canvas. The frontend is unaffected, so the editor and the rendered page disagree about where the composition starts.

Change

Reserve no height for a picker that is withheld. The placeholder keeps its normal-flow bounding (position:relative; inset:auto) so it cannot escape the canvas, and takes min-height: 0 so it displaces nothing.

Verification

• 292 parity fixtures pass unchanged.
• Contract coverage asserts both halves of the contract together: the picker is withheld, and no height is reserved for it.

AI assistance disclosure

Implemented with AI assistance: Claude Sonnet 4.5 running in the opencode CLI agent. The AI measured the placeholder geometry in a live editor canvas, identified the reserved height as the displacement source, made the change, and ran the suite. A human reviewed and directed the work.