Fix cell/outline breakage after single-line display math with attributes

[max-nothacker]

Fixes #976

Problem

A single-line display math expression with a cross-reference label, e.g.

$$1+1$$ {#eq-spec0}

breaks the extension for the rest of the document: headings disappear from the Outline, LaTeX preview buttons vanish, and later code cells lose their Run Cell button ("Editor selection is not within an executable cell").

Root cause

In math_block() (packages/core/src/markdownit/math.ts), the single-line close check was

if (firstLine.trim().slice(-2) === "$$") {

which fails when an attribute block trails the closing $$. The rule then falls through to the multi-line scan looking for /^\$\$\s*(\{.*\})?\s*$/ on a later line — which never matches — so the loop runs to the end of the document and the math token swallows everything after the equation. Nothing downstream (headings, fences) is tokenized again. Full analysis in #976 (comment).

Fix

Recognize an optional trailing attribute block in the single-line case, mirroring the multi-line close regex, and store it in token.info the same way the multi-line path does (so single-line labeled equations also pick up the math preview decoration):

let attrStr = undefined;
const singleLineMatch = firstLine.trim().match(/^(.*)\$\$\s*(\{.*\})?\s*$/);
if (singleLineMatch) {
  firstLine = singleLineMatch[1];
  attrStr = singleLineMatch[2];
  found = true;
}

Unlabeled single-line math ($$1+1$$) takes the same path it did before ((.*) is greedy, so interior $$ behave as with the previous slice(-2) check), and the multi-line branch is untouched.

Tests

Added packages/core/test/markdownit-math.test.ts (same harness as the existing markdownit-divs.test.ts):

1. single-line labeled equation followed by a {python} fence → one math_block with info === "{#eq-spec0}", fence still tokenized (the Quarto extension display math edge cases stops extension functionality for rest of document #976 symptom)
2. single-line unlabeled equation → unchanged behavior
3. multi-line equation with label on the closing line → unchanged behavior

yarn test in packages/core passes (5/5), and tsc --noEmit is clean.

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