Add CPU throttling option

[marcusquinn]

Summary

• Add a cpu-throttling-rate action input for Chromium runs.
• Apply Chrome DevTools Protocol CPU throttling before each measured navigation.
• Document the input with a 4x slowdown example.

Verification

• npx wp-scripts lint-js tests/performance/specs/main.spec.ts
• npx tsc --noEmit
• git diff --check
• Local integration: mounted Ultimate-Multisite/ultimate-multisite into WordPress Playground with its existing .github/performance-blueprint.json, set CPU_THROTTLING_RATE=4, and ran npm run test:performance with URLS_TO_TEST=/, TEST_ITERATIONS=1, and TEST_REPETITIONS=1; Playwright reported 1 passed and produced performance metrics for /.

Note: npm run lint still reports existing errors in untouched files (tests/performance/cli/results.js, tests/performance/config/performance-reporter.ts, and tests/performance/playwright.config.ts).

Closes #85

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aidevops.sh v3.15.10 plugin for OpenCode v1.14.41 spent 26m on this as a headless worker.

[gemini-code-assist]

Code Review

This pull request introduces a cpu-throttling-rate input to the GitHub Action, enabling CPU slowdown simulation in Chromium during performance tests. The changes include documentation updates, action configuration, and the implementation of throttling via Playwright's Chrome DevTools Protocol (CDP). Feedback suggests refining the input validation to handle zero consistently and optimizing the code to skip CDP session creation when the throttling rate is 1 or less.

[gemini-code-assist]

The validation logic is inconsistent: an empty string returns 0 (disabling throttling), but an explicit "0" string causes an error. Additionally, the error message 'positive number' technically excludes zero. Allowing 0 as a valid input for 'no throttling' would be more consistent.

Suggested change

	if ( ! Number.isFinite( parsedRate ) || parsedRate <= 0 ) {
	throw new Error( 'CPU_THROTTLING_RATE must be a positive number.' );
	}
	if ( ! Number.isFinite( parsedRate ) || parsedRate < 0 ) {
	throw new Error( 'CPU_THROTTLING_RATE must be a non-negative number.' );
	}

[gemini-code-assist]

A CPU throttling rate of 1 represents no slowdown. Since Playwright provides a fresh page for each test iteration, we can optimize by skipping the CDP session creation and command if the rate is 1 or less.

Suggested change

	if ( rate === 0 || browserName !== 'chromium' ) {
	if ( rate <= 1 || browserName !== 'chromium' ) {