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

q2-preview gains a body-level container plus a title-block component.
Plan-2d doc extended with title-block scope + post-2C-landing
alignment notes.

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

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+---
+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

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+---
+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

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+---
+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

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+---
+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

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+---
+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

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+---
+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

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+---
+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

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+---
+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.