fix(make-pdf): assign body heading ids so --toc anchors resolve (#1689)

[0xDevNinja]

Summary

• make-pdf --toc built a table of contents whose <a href="#toc-N"> anchors and <span data-toc-target="toc-N"> page-number spans referenced element ids (toc-0, toc-1, ...) that no body heading ever carried.
• buildTocBlock minted toc-${i} purely for its own markup; the body headings came straight from marked (<h1>One</h1>, no id) and wrapChaptersByH1 never assigned them. So every TOC link was a dead in-page fragment and Paged.js had no target element to resolve page numbers against.

Fixes #1689.

Fix

New annotateHeadingIds(html) pass:

• Walks H1-H3 headings in document order.
• A heading with no id gets id="toc-N" injected (N = document-order index).
• A heading that already declares an id keeps it; the TOC points at the existing id.
• Returns the rewritten body HTML plus the resolved {level, text, id} list, which buildTocBlock now consumes — so anchors and targets are guaranteed to agree with the body.

The id annotation only runs when --toc is requested (the ids exist solely for the TOC), so non-TOC renders are byte-for-byte unchanged.

Before / after (issue repro)

bun -e '
import { render } from "./make-pdf/src/render.ts";
const r = render({ markdown: "# One\n\n## Sub\n\nbody\n\n# Two\n", toc: true });
const hrefs = [...r.html.matchAll(/href="(#toc-\d+)"/g)].map(m => m[1]);
const ids = [...r.html.matchAll(/<h[1-3][^>]*\bid="([^"]+)"/g)].map(m => m[1]);
console.log("anchors:", hrefs, "heading ids:", ids);
'

Before: anchors: ["#toc-0","#toc-1","#toc-2"] heading ids: []
After: anchors: ["#toc-0","#toc-1","#toc-2"] heading ids: ["toc-0","toc-1","toc-2"]

Tests

3 new cases in make-pdf/test/render.test.ts:

• TOC anchors + data-toc-target all resolve to a real body heading id.
• A heading's pre-existing id is preserved and linked, id-less siblings still get a minted id.
• No id="toc- injection when toc is off.

bun test make-pdf/test/render.test.ts
# 61 pass, 0 fail

[jbetala7]

Collision check: #1690 is already open for the same issue and same files. It was opened on 2026-05-25 by the issue author, also targets #1689, touches make-pdf/src/render.ts and make-pdf/test/render.test.ts, assigns stable TOC ids, preserves existing heading ids, and pins the no-TOC behavior.

This patch looks directionally correct, but functionally duplicate. I would treat #1690 as the canonical PR unless maintainers prefer this branch. If this one continues, it should explicitly explain why #1690 is being superseded.