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/exploration/visual-composition-editing-exploration.md

@@ -149,14 +149,13 @@ DOM `<g name="…">` nodes measurable via `getBoundingClientRect()`. The app alr
 
 ## 6a. Deferred polish (Phase A follow-ons)
 
-Built but parked for a later pass:
-
-- **Mark-type glyph per leaf.** A simplified icon of each unit view's mark (bar/line/point/…)
-  inside its box, so which-is-which reads at a glance without hovering. Needs a small mark→glyph
-  set (its own icon-ledger sub-family).
-- **A more legible `layered` primitive.** Layers currently render as offset stacked rectangles.
-  A dedicated "stacked planes" metaphor — overlapping sheets/disks, like the database glyph's
-  stacked cylinders — would convey "one space, several layers" more elegantly than nudged boxes.
+- **Mark-type glyph per leaf.** _(Shipped.)_ A simplified glyph of each unit view's mark inside
+  its box (the `mark-*` icon sub-family), so which-is-which reads at a glance.
+- **A more legible `layered` primitive.** _(Shipped.)_ A layer renders as **one frame** holding
+  its child marks as a row of glyphs, badged as layered (the `layers` glyph) — not separate boxes,
+  so it reads as one space and stays distinct from a concat (which is box-per-view). The
+  overlapping/stacked-planes options weighed here were rejected: at glyph scale, overlapping
+  line-art muddies the very marks the glyphs exist to show; legibility beat the z-order-depth cue.
 - **Hide the affordance for single-view specs.** _(Shipped.)_ The toolbar glyph appears only when
   the spec has a composition.