feat(browser): opt-in long animation frame and slow interaction spans - #1158

Confidence 4/5 · likely safe to merge
The perf spans reuse the existing tracer, sampler and consent gate, and the browser tests exercise a real slow click and the LoAF fallback.
_{quality 100/100 · no findings · tests covered · risk medium · 2/2 new units observable}

Adds two opt-in browser tracing sources: longAnimationFrame/longtask spans for frames ≥100ms (with script attribution) and interaction <event> spans for interactions ≥200ms, both buffered and nested under the open navigation span. Contained, tested, safe to merge.

• startPerf observes long frames and slow interactions in the deferred chunk
• tracing.longFrames / tracing.slowInteractions config options (default false)
• openNavigationSpan exported so perf spans nest under the navigation
• selectorOf exported from @maple/browser-session for the interaction target

Fixed since the last review

• F1 · captureHeaders JSDoc block now documents longFrames

_{f8588b2 · Updated on every push. Reply "won't fix" to dismiss a finding, or mention @maple to ask about one.}