fix(tests/playwright): batch stale-test cleanups (5 of 10)

[avrabe]

Summary

Five Playwright tests had stale assertions that diverged from the rendered output. None require a code change in rivet-cli or rivet-core — these are pure test-code fixes.

• artifacts.spec.ts:49 — mermaid race. The test asserted toContainText("flowchart LR") on <pre class=\"mermaid\"> after mermaid.js had already replaced the source with rendered SVG. Drop the source-text assertion; keep only the SVG-presence check.
• documents.spec.ts:145 — heading IDs selector too broad. article h2, article h3, article h4, main h2, main h3, main h4 matched the page-chrome <h2>{title}</h2> (rendered by rivet-cli/src/render/documents.rs:122 with no id by design). Tighten to .doc-body h2-h4 so only body headings (which correctly emit id=\"{slug}\" via rivet-core/src/document.rs:541) are checked.
• rivet-delta.spec.ts:136 — strict-mode locator violation. page.locator(\"code\", { hasText: \"REQ-NEW-1\" }) matched both <code class=\"language-mermaid\"> (mermaid source contains REQ_NEW_1) and the standalone <code>REQ-NEW-1</code> row. Tighten to code:not(.language-mermaid) plus .first().
• rivet-delta.spec.ts:258 — setContent has no base URL. <script src=\"/assets/mermaid.js\"> resolved to about:blank/... and never loaded → window.mermaid undefined → 30s timeout. page.goto(\"/\") before setContent so the script resolves against the dev server.
• rivet-delta.spec.ts:367 — missing image regex in mdToHtml helper. The local mdToHtml only had a [link](url) regex, which matched the inner [alt](url) of ![alt](url) and produced !<a href=\"url\">…</a> instead of <img>. Add a ![alt](url) regex BEFORE the link regex.

The other 5 Playwright failures (artifacts.spec.ts:73, filter-sort.spec.ts:225/253, serve-variant.spec.ts:25/67) are addressed in separate PRs (serve middleware fix, decision-pending, js.rs reload restoration).

Test plan

• CI Playwright job: the five tests at artifacts.spec.ts:49, documents.spec.ts:145, rivet-delta.spec.ts:136, rivet-delta.spec.ts:258, rivet-delta.spec.ts:367 pass.
• No file outside tests/playwright/ is touched (verify via git diff --stat).
• The other 5 known-failing tests still fail (out of scope for this PR; tracked elsewhere).

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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