Docs: reconcile BunPress 0.2 output manifest

Parent: #494 Related: docs/DOCS_ACCURACY_PLAN.md

Problem

bun run docs:build currently renders successfully through the owner-maintained ../../Tools/bunpress checkout, then fails its output-manifest gate because the renderer identity changed from BunPress 0.1.21 to 0.2.0. The new tree adds search-index.json and changes every rendered page, so this is a renderer-wide release boundary rather than an incidental docs-data update.

Observed on 2026-08-10 from clean tracked zig-js main after #526:

• manifest generator: 0.1.21
• active owner renderer: 0.2.0
• added output: search-index.json
• changed output: every documented HTML page, with substantial byte/hash deltas

Scope

• Audit the owner BunPress 0.2.0 change and rendered output for intended markup, assets, search-index content, links, escaping, and deterministic ordering.
• Resolve defects in ../../Tools/bunpress, not through a zig-js-local renderer substitute.
• Regenerate docs/.data/bunpress-output-v1.json only after the complete rendered-tree diff is reviewed and stable.
• Make bun run docs:build pass from clean tracked zig-js and BunPress inputs and keep CI reproducible.

No-workaround rules

• No bypass of --check-manifest, stale-version spoof, selective hash omission, committed dist/, or blanket manifest acceptance without inspecting the renderer-wide change.
• Do not conflate unrelated source/evidence edits with the renderer upgrade.

Acceptance

• BunPress 0.2.0 output and search-index.json are audited for correctness, determinism, and safe escaping.
• Any renderer defect is fixed and tested in the owned BunPress source.
• bun run docs:manifest records only the accepted final tree and the exact generator identity.
• Two clean consecutive bun run docs:build runs reproduce the manifest byte-for-byte.
• The resulting zig-js commit reports flips 0 test262 cases and links the exact BunPress revision used.

Completed and landed on main.

• Owner renderer fix: stacksjs/bunpress@ae928ff unifies rendered headings, search records, and SEO checks on one slug-reservation path; it covers punctuation, numbered headings, Markdown markup, duplicates, and custom-anchor collisions.
• Accepted zig-js evidence: f98a34a2 records BunPress 0.2.0, 49 pages, 53 outputs, and tree SHA-256 d301b1cf2a2c8684ed2ac008d0bdc1883d0e52e1e55b4c90c7d3f067f1e19170.

Audit results:

• 357 unique search records; maximum stored section text is the configured 800 characters.
• Every indexed fragment resolves against the corresponding rendered HTML page; 0 missing pages, 0 missing anchors, and 0 duplicate URLs.
• Search rendering escapes query, URL, breadcrumb, title, and preview text before its controlled match-highlighting markup is inserted.
• Two independent output directories produced byte-identical manifests, and two subsequent bun run docs:build runs passed the checked-in manifest gate.
• BunPress: 504/504 tests and TypeScript checking pass.
• Flips 0 test262 cases.