Add screenshot capture with configurable always/on_error/never modes

[bigtiger]

Summary

• Adds a screenshots: option to Perchfall::Client#run, accepting :always, :on_error (default), or :never
• Captures a PNG via Playwright and embeds it as base64 in the report under report.screenshots
• Excludes screenshot data from to_h/to_json by default; opt-in via include_screenshots: true to prevent accidental leakage of sensitive page content into logs and serialised payloads

Details

The Node script accepts a --screenshot flag and returns the captured PNG as base64 in the JSON output under the screenshots key. On the on_error path, capture runs inside a .catch(() => null) guard so a screenshot failure never obscures the underlying page error.

The safe-by-default serialisation decision mirrors the existing SSRF protections: screenshot data from authenticated pages can contain session tokens, PII, or internal tooling state. Callers must explicitly pass include_screenshots: true to include it in output.

See ADR 0016 for full rationale, transport overhead considerations, and the screenshot_path: file-based transport flagged as future work.

Test plan

• bundle exec rspec passes (194 examples, 0 failures, 100% line coverage)
• screenshots: :always — verify report.screenshots is a non-nil base64 string on a successful check
• screenshots: :on_error — verify report.screenshots is nil on a clean check and non-nil when the page fails
• screenshots: :never — verify report.screenshots is always nil
• report.to_h excludes screenshots key by default
• report.to_h(include_screenshots: true) includes the key
• report.to_json(include_screenshots: true) round-trips correctly through JSON.parse
• Invalid screenshots: value raises ArgumentError with a clear message