Add addon overhead benchmark baseline

[mattcosta7]

Summary

• add a dedicated headless-Chromium Vitest benchmark project for addon overhead
• compare identical lifecycle, raw DOM mutation, and React commit work across addon disabled, panel hidden, and panel visible states
• emit ignored, comparison-ready JSON and document the top-level benchmark and --compare commands
• run the benchmark in the existing browser CI job and upload its JSON without enforcing timing thresholds
• pin Vitest and its Playwright provider to 4.1.10 while benchmark support is experimental

The CI benchmark is a functional check only. Its measurements remain diagnostic until repeated runs establish stable variance and defensible budgets.

Validation

• npm run benchmark
• npm run benchmark -- --compare <baseline.json>
• npm run tsc
• Chromium lifecycle/collector slice: 61 tests passed
• npm run lint (passes with one existing warning in the forced-reflow collector test)

No changeset is included because this only adds development and CI measurement infrastructure.

[Copilot]

Pull request overview

Adds diagnostic browser benchmarks to establish addon overhead baselines.

Changes:

• Benchmarks lifecycle, DOM mutation, and React workloads across three addon states.
• Adds benchmark commands, documentation, JSON output, and CI artifact upload.
• Pins experimental Vitest benchmark dependencies to 4.1.10.

Show a summary per file

File	Description
.github/workflows/ci.yml	Runs and uploads benchmark results.
.gitignore	Ignores generated benchmark JSON.
CONTRIBUTING.md	Documents benchmark usage.
package.json	Adds the root benchmark command.
package-lock.json	Records exact Vitest versions.
packages/storybook-addon-performance-panel/package.json	Adds workspace command and pins dependencies.
packages/storybook-addon-performance-panel/vitest.benchmark.config.ts	Configures headless Chromium benchmarks.
packages/storybook-addon-performance-panel/__benchmarks__/overhead.browser.bench.tsx	Implements overhead workloads and scenarios.

Review details

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• Files reviewed: 6/8 changed files
• Comments generated: 0
• Review effort level: Balanced