test(chat): check the renderer warning's anchor against the guide it links to - #3602

Companion to veryfront/veryfront-docs#371. Dogfooding round 2, finding 8.

The finding, and why the previous fix did not close it

A chat surface without a Markdown renderer logs a warning whose only escape hatch is a deep link. Round 1 fixed the path half of that link -- /docs/guides/chat-ui was a genuine 404, and /docs/code/guides/chat-ui is not. Verification against published 0.1.1229 showed the symptom had only half moved:

$ curl -s https://veryfront.com/docs/code/guides/chat-ui | grep -c render-markdown-in-chat
0

The fragment names no heading on the published page. Unlike a bad path, a bad fragment does not 404 -- the browser silently leaves the reader at the top of the article -- so nothing surfaced it.

The framework's string is not wrong. docs/guides/chat-ui.md, the file that page is published from, has carried ## Render Markdown in chat since #3358. The published page is a 2026-08-02 snapshot that predates it, because veryfront-docs' sync workflow has failed on every run since (details in the docs PR). The content fix belongs in veryfront-docs, and is there. Nothing about the emitted URL needs to change here.

What this changes

The test guarding that link, only.

It asserted the whole URL against a second copy of the same string:

assertEquals(links, ["https://veryfront.com/docs/code/guides/chat-ui#render-markdown-in-chat"]);

That can catch someone editing the constant. It cannot catch someone renaming the heading the fragment names -- the exact drift this finding is about -- because the heading is not part of what it compares. Its doc comment claimed the fragment "has to track the heading in this repo's docs/guides/chat-ui.md", but nothing enforced it.

So resolve the published URL back to the repository file it is published from, and assert the fragment against that file's headings. Same shape as the check #3589 added for error docs links.

Confirmed it fails for the right reason by renaming the heading to what the published page currently has:

AssertionError: no "#render-markdown-in-chat" heading in .../docs/guides/chat-ui.md.
A reader following the warning lands at the top of the article instead of the section.
Headings present: prerequisites, add-the-preset-ui, ..., render-markdown-directly, ...

The doc path resolves from import.meta.url, not the process cwd: test files share one process under --parallel and src/testing/cwd.ts chdirs it. lint:cwd-relative-test-reads passes with no new baseline entry.

Scope

No runtime change, so nothing to verify against a build. The published-artifact proof for this finding is the live page, and it is recorded on the docs PR:

https://veryfront.com/docs/code/guides/chat-ui#render-markdown-in-chat must scroll to a "Render Markdown in chat" heading once veryfront/veryfront-docs#371 is live.

Not fixed here

veryfront-docs' update-reference sync has failed on every run since 2026-08-02, so no code doc merged since then has reached the site. Its quality gate runs after the copy and before the PR, so one non-compliant file blocks delivery of all of them, and the 13 issues it reports live in docs/** in this repository. That is very likely why several round-1 doc fixes never appeared live. It needs its own issue; the offending files are other findings' territory.