fix(make-pdf): give TOC headings stable ids so --toc anchors and page numbers resolve

[officialasishkumar]

Summary

make-pdf --toc built a table of contents whose entries linked to, and whose page-number spans targeted, element ids (toc-0, toc-1, ...) that were never assigned to any heading. Every TOC anchor was a dead in-page link and every page-number target was unresolvable, so Paged.js had nothing to fill the .toc-page spans against.

Fixes #1689.

Current behavior on upstream main

On origin/main (920a13a1):

TOC anchor hrefs:    [ "#toc-0", "#toc-1", "#toc-2" ]
heading ids in body: []

The TOC entries point at #toc-0 / #toc-1 / #toc-2, but no heading in the body carries any of those ids (they come straight from marked and wrapChaptersByH1, neither of which assigns ids).

Fix

Replace extractHeadings with assignTocIds, which walks the H1-H3 headings in document order, assigns id="toc-N" to each, and returns both the id annotated body HTML and the heading list. render() uses the annotated HTML for the body when a TOC is requested and builds the TOC from the same list, so anchors and data-toc-target ids resolve to real headings. A heading that already declares an id keeps it (the TOC points at that id), so no duplicate id is emitted.

With no TOC the body is untouched (no ids injected), so non-TOC output is byte-identical to before.

Validation

After the fix:

hrefs: ["#toc-0","#toc-1","#toc-2"] | heading ids: ["toc-0","toc-1","toc-2"] | all resolve: true

Tests (the make-pdf-gate unit step, bun test make-pdf/test/*.test.ts):

90 pass, 0 fail

Three new regression tests cover: anchors and page targets resolving to heading ids in document order, reuse of an existing heading id without duplication, and no id injection when the TOC is off.

The P0 combined-features gate (make-pdf/test/e2e/combined-gate.test.ts) runs generate ... --quiet with no --toc. Since the change is entirely behind if (opts.toc) and --toc defaults to false, the gate's rendered HTML (and extracted text) is unchanged.

Scope

make-pdf/src/render.ts only (render, buildTocBlock, and extractHeadings to assignTocIds), plus three regression tests. No template, VERSION, or CHANGELOG changes.