Adopt INP as Core Web Vital, deprecate FID

[m0nkey]

Problem

INP replaced FID as the responsiveness Core Web Vital in March 2024, and FID was removed from Chrome tools by September 2024. The Playwright probes still only collect FID, missing the metric that actually matters for responsiveness monitoring.

Solution

Add INP collection via a PerformanceObserver init script injected before page creation. INP is reported as browser.performance.inp on document spans. FID stays for backward compatibility but is marked deprecated and recommended for removal.

Notes

• The observer tracks worst-case interaction duration rather than p98 like web-vitals; this is reasonable for synthetic probe interactions.
• INP will be null on probes that don't include synthesized interactions (click/type). That's expected.
• FID has no removal date set; may want a follow-up to drop it in a future version.

Test plan

• Existing Playwright probe tests pass
• New tests: init script injection + INP span attribute
• Integration: probe with interactions confirms browser.performance.inp on the document span

Pull request

#21

[lewispb]

I like the idea but will have a think about the best solution.