Equation numbering as a format-specific post-filter stage (bd-vlhi2zkj)

[cscheid]

Phase 1 of claude-notes/plans/2026-09-21-equation-numbering-and-mathml.md (strand bd-vlhi2zkj). Independent of quarto-math; lands on main first so the stacked MathML PRs on #706 can build on it.

Why

CrossrefRenderTransform appended \tag{N} to every numbered equation's TeX. \tag is an amsmath command only MathJax and KaTeX read: Quarto 1 emits it solely for those two engines and writes \qquad(N) otherwise, and any converter that reads math alone (Pandoc's, quarto-math's for the coming html-math-method: mathml) rejects the whole expression. The encoding of the number is a presentation decision, so it leaves the format-agnostic transform.

What changes

• Crossref-render leaves the Math text byte-identical and records the number on the reserved span attribute quarto-eq-number (EQ_NUMBER_ATTR in crossref/mod.rs).
• New EquationNumberStage, placed after UserFiltersStage::post and before CodeHighlightStage, encodes the number from the format and html-math-method: \tag{N} for MathJax/KaTeX, \qquad(N) for plain/unknown, a sibling span.quarto-eq-number outside the math (plus a quarto-eq-sibling-number modifier class) for mathml, nothing for non-HTML formats. It removes the attribute in every case. Running after the post filters is the point: a Lua filter can read, rewrite or delete el.attributes["quarto-eq-number"] — documented in the Lua filters guide as the supported way to customize equation numbers.
• math_method.rs parses html-math-method once (string and object forms); MathEngine::from_meta in math-js maps over it so the two stages cannot drift.
• equation-number.scss, a shared SCSS layer loaded at every HTML compile site and in assemble_reveal_scss (like copy-code.scss), lays the sibling label out at the right edge of the equation row.
• The pipeline design doc gains a "post-filter presentation slot" section; Equation.tsx's comment points at the stage (the preview keeps its own KaTeX \tag append).

Tests

Written first. Crossref-render unit tests assert the attribute instead of the tag; 13 stage unit tests (encodings, selection, walker); 13 end-to-end tests in equation_numbering_pipeline.rs covering every encoding, revealjs, KaTeX, the object form, unlabelled math, sequential numbering, and Lua post filters rewriting/deleting the number plus a pin that pre filters never see it; quarto-sass asserts the layer in the HTML and revealjs CSS.

Collateral: the HTML stage-list assertions (25 → 26 stages) and the styles.css byte-identity baseline (tests/fixtures/phase5-single-doc-baseline/expected_hashes.txt, re-captured with a dated note; the layer is additive, doc.html unchanged).

Verification

Full cargo xtask verify green. End-to-end through cargo run --bin q2 -- render (recorded in the plan): default → \tag{1} inside the math with MathJax loaded; plain → \qquad(1); mathml → <span class="quarto-eq-number">(1)</span> after the math and no engine loaded.

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