Feature rune layout axis and responsive collapse

Motivation

feature currently fuses three orthogonal axes into one tag: section framing (eyebrow / headline / blurb preamble + an optional media slot in a split layout), item identity (the definition list — icon/title/description triplets), and item arrangement (grid vs. flat column). The third axis has no attribute of its own. Instead, arrangement is inferred from media placement, in plugins/marketing/src/config.ts:

variants: {
  'media-position': {
    top:    { staticModifiers: ['definitions-grid'] },
    bottom: { staticModifiers: ['definitions-grid'] },
  },
},

"Media stacked → tile as grid; media beside → stack in a column" is a reasonable default, but wiring two orthogonal concerns to one dial is exactly what makes the rune hard to reason about, and it makes some combinations unreachable (e.g. media beside content and a grid of items). Promoting arrangement to a first-class layout axis decouples the two: media-position controls where the media sits, layout controls how the items lay out, and they become independent.

This is also the first real consumer of the ADR-018 canonical pool — feature needs no bespoke values, only grid and list.

Current state

• feature (plugins/marketing/src/tags/feature.ts) is a page-section built on SplitLayoutModel (splitLayoutAttributes in packages/runes/src/tags/common.ts). Its body content model is media --- content, where content resolves eyebrow, headline, blurb, and a definitions list.
• The definitions render to a flat <dl> carrying an item count. Whether that <dl> tiles as a grid or stacks is toggled today by the definitions-grid static modifier, driven by the media-position variant above (SPEC-091).
• feature already has a collapse attribute — inherited from splitLayoutAttributes (common.ts:24): "Breakpoint at which side-by-side layouts collapse to a single stacked column." It governs the media↔content split (sm | md | lg | never), surfaced via buildLayoutMetas. It does not today govern the item arrangement.

Design

1. layout axis (grid | list)

Add a layout attribute to feature, with values drawn from the ADR-018 canonical const:

• layout="grid" — feature-items tile as a grid (today's "definitions-grid" look).
• layout="list" — feature-items stack in a single column (today's "beside-media" look). Note this is the canonical list token, not a bespoke columns value — feature adds no new vocabulary to the pool.

The engine sets data-layout from the layout meta modifier (the same mechanism gallery already uses), and CSS keys the grid-vs-stack arrangement off [data-layout="grid"] / [data-layout="list"] instead of the definitions-grid modifier class.

Default: preserve today's behaviour as the default resolution so existing content doesn't shift — when layout is unset, derive it from media-position (stacked media → grid, beside media → list). The difference from today is that the author can now override that default explicitly, and the two axes are independent when they do.

2. Retire the media-position → definitions-grid coupling

Remove the variants: { 'media-position': { … } } block and the definitions-grid static modifier from the Feature config. The grid/stack decision moves entirely onto [data-layout]. The media-position-derived default from §1 replaces the old hard coupling, so the rendered output for existing content is unchanged when layout is unset — but this is a behaviour change with visual-regression surface and is owned by this spec, not a silent side effect. CSS in packages/lumina/styles/runes/feature.css (and the definitions-grid selector) is updated accordingly.

3. Resolve the collapse collision

feature already has a collapse (the media-split breakpoint, §Current state). The item layout also needs a responsive form — on a narrow viewport a grid should reflow to a single stacked column. These are two orthogonal axes (media↔content vs item↔item) that share the word and the sm|md|lg|never vocabulary.

Decision (this spec): reuse the single collapse breakpoint for both. Below the collapse breakpoint the media stacks under the content and a grid layout reflows to a single column — one dial, one mental model, which is the behaviour authors almost always want anyway. We do not introduce a second layout-collapse attribute. The item-layout reflow is CSS-only (a media query that flattens the grid track), requiring no new behavior code and no new meta. If a real case later demands independent control, a distinct dial can be added without breaking this default — but it is a non-goal here.

Because carousel is out of scope, the collapsed item form in this spec is always a single stacked column. The "collapse to a carousel" target is deferred to SPEC-100, which builds on this collapse semantics.

Acceptance Criteria

• feature accepts a layout attribute matching grid | list, with both values imported from the ADR-018 canonical const (no bespoke columns value).
• The engine emits data-layout from the layout modifier; feature.css keys grid-vs-stack arrangement off [data-layout="grid"] / [data-layout="list"].
• When layout is unset, it defaults from media-position (stacked → grid, beside → list) so existing content renders identically; an explicit layout overrides that default.
• The media-position → definitions-grid variant coupling and the definitions-grid modifier are removed from the Feature config and CSS; the grid/stack decision lives solely on [data-layout].
• Item arrangement and media placement are independently controllable (e.g. media beside content and layout="grid" is reachable).
• Below the existing collapse breakpoint, a grid layout reflows to a single stacked column (CSS-only); no second collapse attribute is introduced.
• CSS-coverage tests pass for the new [data-layout] selectors; the definitions-grid selector is removed from KNOWN_*/coverage as appropriate.
• feature rune docs document layout and the shared-collapse semantics with an example; the media-position-derived default is noted.
• Tests cover: explicit layout overriding the media-derived default, independence of the two axes, and unchanged output when layout is unset.

Non-goals

• Carousel (layout="carousel") and collapse-to-carousel — deferred to SPEC-100.
• A second, independent layout-collapse breakpoint — the single collapse dial is reused; a separate dial is only revisited if a concrete case demands it.
• Letting feature host arbitrary layout primitives (e.g. a bento) in place of feature-items — that belongs to a generic section primitive, not feature (out of scope; noted for the record).
• Changes to the canonical pool itself or to other runes' layout values — owned by ADR-018 (lazy migration).

References

• ADR-018 — canonical layout vocabulary this draws grid/list from.
• SPEC-100 — carousel as a shared layout mode (the deferred third value).
• SPEC-091 — the media-position definitions-grid variant being retired.
• plugins/marketing/src/tags/feature.ts, plugins/marketing/src/config.ts (Feature), packages/runes/src/tags/common.ts (splitLayoutAttributes, buildLayoutMetas), packages/lumina/styles/runes/feature.css.