fix(serve): mermaid fenced blocks broken by HTML-entity escaping

[avrabe]

Summary

User-reported bug (v0.6.0 + v0.8.0): a \``mermaidfenced block inside a YAML artifact description renders-->as-->in the dashboard's

`, so mermaid.js fails with "Syntax error in text" and the diagram never appears.

Root cause: render_markdown (rivet-core/src/markdown.rs) injects synthetic <pre class="mermaid"> wrappers but still passes the fenced body to pulldown-cmark's html::push_html, which HTML-entity-escapes Event::Text — so --> → --&gt;, <|-- → &lt;|--, etc. Mermaid diagram syntax is full of </>/&, so this breaks essentially every diagram type.
Fix: re-tag the in-mermaid Event::Text segments as Event::Html so they pass through push_html verbatim. Diagram source contains no HTML tags; a </pre> smuggled into a \``mermaidblock would at worst close the block early, and<script>etc. are still stripped by the existingsanitize_html pass. (document.rs's separate document renderer already emits the body verbatim — only render_markdown` had the bug.)
Verified locally: rivet serve → /artifacts/ARCH-CORE-001 now contains Config --> Store verbatim, zero --&gt;.
Test coverage

markdown.rs unit tests (×2): stateDiagram [*] --> arrows survive verbatim with no --&gt;; classDiagram <|-- survives with no &lt;|--.
tests/playwright/mermaid.spec.ts (new): raw HTML of /artifacts/ARCH-CORE-001 has Config --> Store verbatim and no --&gt;; mermaid.js renders it to an <svg> with no "Syntax error" box.

Follow-up (not in this PR)
The fix is diagram-type-agnostic (it just stops escaping the body), so it covers flowchart/state/class/sequence/ER/gantt/etc. at once. A broader Playwright matrix that exercises one fixture artifact per diagram type would be nice belt-and-suspenders but is its own task — flagging per the request to "test all kinds of mermaid graphics".
Test plan

 cargo test -p rivet-core markdown green (incl. 2 new tests)
 Playwright mermaid.spec.ts green in the E2E job
 No regression in existing markdown / mermaid tests

🤖 Generated with Claude Code

[github-actions]

📐 Rivet artifact delta

No artifact changes in this PR. Code-only changes (renderer, CLI wiring, tests) don't touch the artifact graph.

[github-actions]

⚠️ Performance Alert ⚠️

Possible performance regression was detected for benchmark 'Rivet Criterion Benchmarks'.
Benchmark result of this commit is worse than the previous benchmark result exceeding threshold 1.20.

Benchmark suite	Current: 90bb1a1	Previous: 02b9328	Ratio
validate/10000	20318603 ns/iter (± 1714130)	16904098 ns/iter (± 1013276)	1.20
query/10000	138521 ns/iter (± 410)	91811 ns/iter (± 267)	1.51

This comment was automatically generated by workflow using github-action-benchmark.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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