q2-preview: body container + title block (Plan 2D)

Phase 6.0 framework extraction:
- Lift `inlinesToPlainText` / `blocksToPlainText` to `framework/plainText.ts`.
- Add `framework/meta.ts` with `extractMetaString` / `extractMetaBool` /
  `extractMetaStringList`.
- Flat barrel re-exports on `framework/index.ts` (plainText, meta,
  customNode bookkeeping fix).
- Tighten `FormatRegistry` at `framework/types.ts` with typed optional
  entries for the two synthetic keys (`__fallback__`, `__title_block__`).
- Migrate `ReactAstSlideRenderer.tsx`'s private `extractMetaString` to the
  framework helper. Deliberate behavior change for slide titles/authors
  containing inline markup (Emph/Strong/Code/Link now render as text
  instead of empty string) locked by a regression test.
- Consolidate `meta.format` extraction in `ReactRenderer.tsx` and
  `getQ2Format.ts` onto the framework helper.
- Expose framework helpers on `window.__Q2_PREVIEW_RENDERER__` for
  user TSX overrides.

Phase 6.1+ body container in `PreviewDocument.tsx`:
- Page-layout-aware `<div id="quarto-content"><main class="content">`
  wrapper mirrors Rust template at `template.rs:185-247`.
- Imperative `document.body.className` management via useEffect with
  cleanup-on-unmount.
- Minimal-mode title synthesis re-implements
  `transforms/title_block.rs:54-110`'s minimal-mode branch React-side
  (the transform is in `Q2_PREVIEW_TRANSFORM_EXCLUDED`).
- Iframe `document.title` wiring from `meta.pagetitle ?? meta.title`,
  only when non-empty (preserves the static `q2-preview.html` title for
  untitled docs).

Phase 7 title block:
- New `q2-preview/custom/PreviewTitleBlock.tsx` (~110 LOC) mirrors Rust
  template at `template.rs:211-240` byte-for-byte. Multi-author and
  date-without-author Rust quirks deliberately replicated to avoid
  per-format CSS forks.
- Registered under synthetic `__title_block__` key in `previewRegistry`;
  mounts inside `<main>` ahead of body blocks. `AstProps` shape parallels
  the `Ast` key (not `NodeArgs`).
- Built-in exposed on `__Q2_PREVIEW_RENDERER__` so user TSX overrides
  can compose (rather than fully replace) the chrome.

Tests:
- 27 unit tests for framework/meta.ts + framework/plainText.ts
- 18 integration tests for PreviewDocument (body container, body classes,
  minimal-mode title synthesis, iframe document.title)
- 17 integration tests for PreviewTitleBlock (required elements,
  Pandoc-falsy semantics, user override full-replacement + composition)
- Synthetic-key registration directly asserted in registry.test.ts (closes
  a pre-existing gap on `__fallback__`).

Smoke-all q2-preview fixtures (11 new):
- body-container-default / -full-layout / -minimal / -minimal-title
- body-classes-override / -full-layout-combo
- title-block-default / -full / -no-title / -multi-author / -date-no-author

All 11 fixtures green in Playwright e2e; full plan checklist (19 items)
ticked; cargo xtask verify clean.

Author: gordonwoodhull

claude-notes/plans/2026-05-10-q2-preview-plan-2d-body-container.md

@@ -0,0 +1,22 @@
+---
+title: Body classes + full layout combo
+format: q2-preview
+body-classes: custom-cls
+page-layout: full
+_quarto:
+  tests:
+    run:
+      requires_js: true
+    q2-preview:
+      ensureHtmlElements:
+        -
+          - 'div#quarto-content.page-layout-full'
+          - 'body.custom-cls'
+        -
+          - 'body.fullcontent'
+          - 'div.page-layout-article'
+---
+
+A paragraph; both knobs are set together to lock the case where a
+future refactor might conflate the body-classes useEffect with the
+page-layout className.

crates/quarto/tests/smoke-all/q2-preview/body-classes-full-layout-combo.qmd

@@ -0,0 +1,18 @@
+---
+title: Body classes override
+format: q2-preview
+body-classes: custom-cls
+_quarto:
+  tests:
+    run:
+      requires_js: true
+    q2-preview:
+      ensureHtmlElements:
+        -
+          - 'body.custom-cls'
+        -
+          - 'body.fullcontent'
+---
+
+A paragraph; the iframe body carries the user `body-classes` override
+and does NOT also carry the literal `fullcontent` default.

crates/quarto/tests/smoke-all/q2-preview/body-classes-override.qmd

@@ -0,0 +1,17 @@
+---
+title: Body container default
+format: q2-preview
+_quarto:
+  tests:
+    run:
+      requires_js: true
+    q2-preview:
+      ensureHtmlElements:
+        -
+          - 'div#quarto-content.page-layout-article'
+          - 'main.content#quarto-document-content'
+          - 'body.fullcontent'
+        - []
+---
+
+A paragraph in the default-layout body container.

crates/quarto/tests/smoke-all/q2-preview/body-container-default.qmd

@@ -0,0 +1,18 @@
+---
+title: Body container full layout
+format: q2-preview
+page-layout: full
+_quarto:
+  tests:
+    run:
+      requires_js: true
+    q2-preview:
+      ensureHtmlElements:
+        -
+          - 'div#quarto-content.page-layout-full'
+          - 'main.content'
+        -
+          - 'div.page-layout-article'
+---
+
+A paragraph rendered inside the `page-layout-full` wrapper.

crates/quarto/tests/smoke-all/q2-preview/body-container-full-layout.qmd

@@ -0,0 +1,23 @@
+---
+title: Doc
+format: q2-preview
+minimal: true
+_quarto:
+  tests:
+    run:
+      requires_js: true
+    q2-preview:
+      ensureHtmlElements:
+        -
+          - 'h1'
+          - 'p'
+        -
+          - 'div#quarto-content'
+          - 'main.content'
+          - 'header#title-block-header'
+---
+
+A paragraph; with `minimal: true` and a `title`, q2-preview synthesizes
+a `<h1>Doc</h1>` ahead of the body (mirrors Rust's title-block
+transform's minimal-mode branch). The full title-block chrome is
+suppressed by the minimal wrapper-skip branch.

crates/quarto/tests/smoke-all/q2-preview/body-container-minimal-title.qmd

@@ -0,0 +1,19 @@
+---
+format: q2-preview
+minimal: true
+_quarto:
+  tests:
+    run:
+      requires_js: true
+    q2-preview:
+      ensureHtmlElements:
+        -
+          - 'p'
+        -
+          - 'div#quarto-content'
+          - 'main.content'
+          - 'h1'
+---
+
+A paragraph. The wrapper is skipped under minimal mode, and no synthetic
+h1 is added because no title is set in the metadata.

crates/quarto/tests/smoke-all/q2-preview/body-container-minimal.qmd

@@ -0,0 +1,21 @@
+---
+title: Doc
+date: "2026-05-10"
+format: q2-preview
+_quarto:
+  tests:
+    run:
+      requires_js: true
+    q2-preview:
+      ensureHtmlElements:
+        -
+          - 'header#title-block-header'
+        -
+          - 'div.quarto-title-meta'
+          - 'div.quarto-title-meta-date'
+---
+
+A paragraph; with `date` but no `author`, the Rust template's
+`$if(date)$` block is gated inside the `$if(author)$` branch so the
+date is suppressed. Locked here so the Rust quirk and q2-preview
+mirror each other.

crates/quarto/tests/smoke-all/q2-preview/title-block-date-no-author.qmd

@@ -0,0 +1,20 @@
+---
+title: Doc
+format: q2-preview
+_quarto:
+  tests:
+    run:
+      requires_js: true
+    q2-preview:
+      ensureHtmlElements:
+        -
+          - 'header#title-block-header.quarto-title-block'
+          - 'h1.title'
+        -
+          - 'p.subtitle'
+          - 'div.quarto-title-meta'
+          - 'div.abstract'
+---
+
+A paragraph; with only `title` set, the title-block chrome contains
+just `<h1 class="title">` — no subtitle, author meta, or abstract.

crates/quarto/tests/smoke-all/q2-preview/title-block-default.qmd

@@ -0,0 +1,25 @@
+---
+title: Doc
+subtitle: Sub
+author: Jane Doe
+date: "2026-05-10"
+abstract: A short summary of the document.
+format: q2-preview
+_quarto:
+  tests:
+    run:
+      requires_js: true
+    q2-preview:
+      ensureHtmlElements:
+        -
+          - 'header#title-block-header'
+          - 'h1.title'
+          - 'p.subtitle'
+          - 'div.quarto-title-meta-author'
+          - 'div.quarto-title-meta-date'
+          - 'div.abstract'
+          - 'div.abstract-title'
+        - []
+---
+
+A paragraph beneath the full title-block chrome.