chore: switch test runner to jsdom

[DylanPiercey]

Switches the test runner from playwright to jsdom.

Usually you'd do the opposite but doing so here gives us:

• deterministic runs (playwright was kinda flakey)
• much faster runs
• much much faster runs in ci since there's no playwright install

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• src/__tests__/fixtures/isomorphic-marko-dep/test.config.ts
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• src/__tests__/main.test.ts
• src/__tests__/utils/create-browser.ts

---

Walkthrough

This PR replaces Playwright-based browser automation with a JSDOM-backed Browser abstraction and a new fromURL() harness that streams and evaluates HTML/modules. The main test harness is refactored to use ephemeral server ports, browser-side HMR tracking (window.__nextHmr and reloadCount), and Browser.drain/settle flows. Fixtures are migrated from async Playwright page APIs to synchronous browser.window.document DOM interactions. CI and dev dependencies remove Playwright, bump jsdom, and enable Mocha VM modules.

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 2.56% which is insufficient. The required threshold is 80.00%.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title 'chore: switch test runner to jsdom' directly and accurately summarizes the main change: migrating the test infrastructure from Playwright to jsdom.
Description check	✅ Passed	The description explains the motivation for switching from Playwright to jsdom and lists concrete benefits (deterministic runs, faster local execution, faster CI).
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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✨ Finishing Touches

📝 Generate docstrings

• Create stacked PR
• Commit on current branch

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch switch-to-jsdom

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