perf: add browser paint phase measurements

[wolfiesch]

Summary

• split the 10k-history browser benchmark into navigation, connection, transcript, real-list, and stable-paint phases
• record renderer milestones inside the browser animation-frame loop, without Playwright polling delay
• keep paint-only probing outside the existing mount metric and report the full instrumented scenario separately
• document the measurement boundary and Playwright TestInfo timing trap

VPS sample

Five corrected runs on the same private Linux VPS produced medians of 520 ms from session click to transcript shell, 812 ms to the real list, and 840 ms to the stable painted tail. The original mount-assertion boundary was 2.95 s; the full instrumented scenario was 3.14 s.

Verification

• pnpm check
• node --test scripts/perf/perf.test.mjs
• five-repetition VPS scripts/perf/ui.mjs run under Node 24

[Copilot]

Pull request overview

This PR enhances the pnpm perf:ui benchmark to emit additional, renderer-clock-based phase timings (navigation → connection → transcript visible → real list visible → stable paint) alongside the existing end-to-end Playwright test duration, and documents how to interpret the new measurements.

Changes:

• Extend the UI perf runner to collect per-phase samples from the browser benchmark and include them as separate metrics in the perf report.
• Instrument the @soak 10k-history mount test to record phase timestamps (including a “two RAFs after tail visible” paint-stability sample) and optionally write them to a file.
• Update performance documentation to explain the new UI phase breakdown and the measurement boundary.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
scripts/perf/ui.mjs	Collects phase JSON output per repetition, validates samples, and emits additional per-phase metrics in the UI perf report.
e2e/remote-app.spec.ts	Records renderer-clock timestamps for navigation/connection/mount/paint milestones and writes phase metrics when requested by the perf runner.
docs/PERFORMANCE.md	Documents the new UI phase timings and clarifies interpretation (renderer clock, not physical display latency).

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