Engine + layout primitives + composable runes + Lumina chip-universal

Acceptance Criteria

• metaFields, zones, sections, zoneLayouts, order accepted on RuneConfig. Types in packages/transform/src/types.ts extended. mergeThemeConfig in packages/transform/src/merge.ts threads the new fields through theme overrides (per-zone replacement semantics, null to suppress, omit to inherit).

• Mutual-exclusion validation at mergeThemeConfig. After merging, walk the resolved config's zones + sections and set-intersect their key names. Any non-empty intersection is a build error naming both the rune and the conflicting position ("{Rune} declares both zones.{slot} and sections.{slot} — pick one source per slot"). Tests in packages/transform/test/merge.test.ts.

• Layout dispatcher with three primitives. Engine path in packages/transform/src/engine.ts resolves each zone via the rune's metaFields manifest, picks the layout from zoneLayouts, and emits DOM per the layout's contract:

  • split → two-slot DOM with data-eyebrow-slot="left" and data-eyebrow-slot="right". Left renders as plain text (primary-color via CSS); right renders as a chip when the field carries a sentimentMap, plain text otherwise.
  • chip-row → wrapping row, every value as a chip.
  • definition-list → <dl> with <dt> + <dd> per field, value rendered as chip when the field carries a sentimentMap, plain text otherwise. data-name="row" wrappers carry display: contents for grid participation. Tests in packages/transform/test/engine-zones.test.ts cover the DOM contract for each layout. All chips emit class="rf-badge" plus the existing data-meta-* attributes.

• data-zone-layout namespaced attribute. Layout dispatcher emits data-zone-layout="…" on zone wrappers (not data-layout, which is already used elsewhere). CSS selectors in Lumina target the namespaced form.

• Canonical-ordering engine path. Engine derives render order from the position vocabulary (eyebrow → title → blurb → metadata → body) when no explicit order: [...] field is declared. Sparse positions (e.g. only eyebrow + body) render without empty wrappers. Custom-order override via order: [...] honored. Tests cover both paths.

• preamble auto-derived CSS wrapper. Engine emits a .rf-{block}__preamble wrapper around the header region (eyebrow + title + blurb + metadata when any are declared) so themes get a single CSS hook around the header for spacing / padding rules. Plugins don't declare preamble explicitly.

• Custom-position escape hatch. A rune declaring a non-vocabulary position name via order: [...] + zones/sections renders with an auto-derived .rf-{block}__{name} CSS class. No theme-default layout applies; the rune is on the hook for picking one via zoneLayouts.{Rune}.{custom-name}. Tests cover the custom-position path.

• Backwards-compat shim for legacy slots + structure. Rune configs that still use the v0.16 slots: [...] array and structure: { ... } tree continue to render via matching layout primitives: header-primary → split (when children fit a left/right pattern) or chip-row, header-secondary → chip-row, content → body section. Engine emits a build-time warning on first encounter naming the rune + the migration path. Tests in engine-zones.test.ts cover the shim path.

• {% eyebrow %} composable rune. New core rune at packages/runes/src/tags/eyebrow.ts. Content model splits the body on a top-level --- into left / right. Emits the identical DOM as a projected zones.eyebrow = { left, right } with the split layout. Tests at packages/runes/test/eyebrow.test.ts cover the split parsing, inline-rune composition inside slots, and the standalone-in-prose rendering.

• {% deflist %} composable rune. New core rune at packages/runes/src/tags/deflist.ts. Content model parses a list where each item starts with **Term:** as a <dt> + <dd> pair. Emits the identical DOM as a projected metadata zone with the definition-list layout. Fallback when an item lacks the **Term:** prefix: emit an empty <dt> + the item's full content in <dd> AND a build-time warning naming the line number. Tests at packages/runes/test/deflist.test.ts cover the parsing, inline-rune composition inside <dd>, the empty-<dt> fallback, and the warning emission.

• metaType typography / layout geometry split in Lumina. packages/lumina/styles/dimensions/metadata.css rewritten so [data-meta-type=…] selectors carry only typography hints (monospace for id, tabular nums for quantity / temporal). The bordered-pill geometry comes off these selectors and lives on [data-zone-layout=…] instead. Universal .rf-badge class becomes the chip primitive emitted by all layout paths + the standalone {% badge %} rune. After this change, every meta-bearing rune in the codebase (migrated or shim-pathed) visually converges on the chip look. runes/badge.css consolidated (the override file goes away since the universal base IS the chip).

• Linked-eyebrow CSS preserved. Lumina styles [data-zone="eyebrow"] a with the primary-color underline treatment matching today's hero behaviour. Visually verified on the site-index hero.

• zoneLayouts resolution chain. Lookup order: theme-level zoneLayouts.{zoneName} → per-rune zoneLayouts.{Rune}.{zoneName} (overrides). When neither is set, fall back to a documented default per zone (split for eyebrow, chip-row for metadata in the engine; Lumina's theme config sets definition-list for metadata as its preferred default).

• Plan plugin builds unchanged. The plan plugin's existing slots + structure configs render through the legacy shim identically to today (modulo the chip-look visual change from the metadata.css rewrite). No plan-plugin config changes in this work item — that's WORK-306.

• CSS coverage tests updated. packages/lumina/test/css-coverage.test.ts reflects the new selector surface. Snapshot tests for layout primitives' DOM contracts pass.

• Docs. New page at site/content/extend/theme-authoring/header-zones.md (or similar) explains the metaFields + zones + zoneLayouts model with worked examples (Work, Card, Recipe, Character from the spec body). New rune reference pages for {% eyebrow %} and {% deflist %} with live {% preview source=true %} examples. Type-vs-layout split + chip primitive documented in extend/theme-authoring/metadata-dimension.md (or wherever the metadata dimension currently lives).