Editor: render a layer as one framed row of mark glyphs

A layer is one plotting space with several marks stacked in z-order, so it now renders as a single frame holding its child marks as a row of glyphs, badged with a new 'layers' glyph — instead of offset stacked cards that read as separate spaces and hid the lower glyphs. Each mark stays an individual treeitem, so selection and Alt+up/down z-order reorder are unchanged.

docs/architecture/10-interaction-and-feedback.md

@@ -228,7 +228,10 @@ schematic of the spec's multi-view composition — `CompositionWireframe`, arch
 tabindex**, arrow keys in **logical (document) order** — Up/Down between nodes, Left → parent,
 Right → first child, Home/End, Enter/Space activate — not spatial, since a mixed horizontal/
 vertical layout makes spatial arrows ambiguous. Each leaf carries a glyph of its mark type (the
-`mark-*` icon sub-family, arch 09 §5) so views read apart at a glance. Selecting a box reveals +
+`mark-*` icon sub-family, arch 09 §5) so views read apart at a glance. A `layer` — one plotting
+space with several marks stacked — renders as **one frame** holding its child marks as a row of
+glyphs, badged as layered (the `layers` glyph), rather than the box-per-view of a concat; each
+mark stays an individual `treeitem` so selection and z-order reorder still work. Selecting a box reveals +
 selects that view's source range in the editor (`AppStore.requestRevealView`) but **does not steal
 focus**, so the wireframe stays the active surface while the editor scrolls to follow; the editor
 selection is the single source of truth. The toolbar glyph appears **only for a composed spec** —