Fix multi-line subtitle escaping its CSS class in HTML output

[cderv]

When subtitle uses a YAML literal block scalar (|), the content escapes the .subtitle CSS class in HTML and RevealJS output. The <p class="subtitle"> element ends up empty, with content rendered as sibling <p> tags outside the styled wrapper.

Root Cause

1. YAML | values are parsed by Pandoc as MetaBlocks containing Para elements — even for single-line content, since | always appends a trailing newline
2. Title-block templates wrap $subtitle$ in <p class="subtitle">$subtitle$</p>. When $subtitle$ is MetaBlocks, Pandoc renders it as <p>...</p>, producing nested <p> tags — invalid HTML5
3. Any HTML5-compliant parser (browser or Quarto's deno-dom post-processing) auto-closes the outer <p> when it encounters the inner <p>, so content escapes the .subtitle class

Single-line values (no |) produce MetaInlines and render correctly since there's no <p> wrapping.

Fix

A normalize filter converts MetaBlocks to MetaInlines for the subtitle field using the existing quarto.utils.as_inlines() API, which calls pandoc.utils.blocks_to_inlines() internally. The conversion is surgical — only subtitle is converted. Fields like abstract and description stay as blocks since they're placed in <div> containers where block-level content is valid.

For multi-paragraph | values, blocks_to_inlines joins paragraphs with LineBreak separators, rendering as <br> in HTML. This preserves the newline semantics of | (a folded scalar > would be used to collapse newlines).

The fix runs in normalize.lua for all output formats, though the nested <p> bug only manifests in formats whose templates wrap these fields in <p> elements.

Fixes #13827

[posit-snyk-bot]

✅ Snyk checks have passed. No issues have been found so far.

Status	Scanner	Critical	High	Medium	Low	Total (0)
✅	Open Source Security	0	0	0	0	0 issues
✅	Licenses	0	0	0	0	0 issues

💻 Catch issues earlier using the plugins for VS Code, JetBrains IDEs, Visual Studio, and Eclipse.