fix: polyfill global CSSStyleSheet for mermaid rendering

[pilat]

Why

CI has been flaky on test/mermaid-elk.test.ts with ReferenceError: CSSStyleSheet is not defined — five render tests fail together, but only sometimes (green most local runs, red on CI). It's a race, not a logic bug, and it surfaces now because we don't pin dependencies: a fresh npm install on the runner shifts timing just enough to trip it.

Root cause

@acemir/cssom (jsdom's CSS engine, pulled in transitively by isomorphic-mermaid) reads a bare global CSSStyleSheet while rendering. isomorphic-mermaid swaps window for svgdom's, which never defines that global, so depending on platform and timing the identifier is simply missing → ReferenceError. macOS almost always wins the race; the Linux runners almost always lose it.

Fix

Define CSSStyleSheet once, up front in loadMermaid(), before any diagram renders — using the real class from the same cssom jsdom uses, with a stub fallback if that transitive package ever moves. This is the same shape as the existing window.* shims a few lines below (Error, Math, …) that already paper over the svgdom environment.

Notes

• The race only reproduces reliably on CI, so the real verification is the CI run on this PR going green — locally the suite was already passing.
• No behavior change for the happy path: when the global is already present, the polyfill is a no-op.

[coderabbitai]

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ℹ️ Review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro

Run ID: 043a0b26-23e3-4991-a1b2-eed9fdf8e1b4

📥 Commits

Reviewing files that changed from the base of the PR and between e2220a2 and 8d8c111.

📒 Files selected for processing (2)

• src/acemir-cssom.d.ts
• src/mermaid/render.ts

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch fix/mermaid-cssstylesheet

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