perf: reveal stable transcripts sooner

[wolfiesch]

Summary

• reduce the cold-mount reveal window from eight to four consecutive ready frames
• retain geometry, tail-alignment, and visible-row readiness checks
• add tail-alignment benchmark boundaries and document the four-frame contract

Matched VPS A/B

Ten repetitions per side on the same Node 24 VPS and 10k-history fixture:

Phase	8 frames	4 frames	Change
click to real list, median	734 ms	679 ms	-55 ms
click to real list, p95	901 ms	704 ms	-197 ms
click to stable paint, median	759 ms	721 ms	-38 ms
click to stable paint, p95	922 ms	746 ms	-176 ms

Verification

• pnpm check
• interaction/motion contract: 22/22
• benchmark helper tests: 5/5
• final VPS UI benchmark: 10/10 repetitions
• cold-mount handoff Playwright test: passed

Broad-suite boundary: web/client packages passed. Untouched desktop suites remain environment-blocked locally by an incomplete Electron install and on the VPS by non-hermetic OMP discovery tests that encounter its real /usr/local/bin/omp.

[Copilot]

Pull request overview

Reduces the cold-mount “warm overlay” reveal latency for the transcript timeline by tightening the stability contract from 8 to 4 consecutive ready frames, while preserving the existing readiness gates (geometry stability, tail alignment, and rows-in-view). It also expands UI benchmark phase reporting to include tail-alignment boundaries and updates documentation/tests to codify the new contract.

Changes:

• Reduce REVEAL_STABILITY_FRAMES from 8 → 4 in TranscriptTimeline while keeping the same readiness criteria.
• Add tail-alignment benchmark phases and corresponding Playwright/E2E phase instrumentation.
• Update performance documentation and interaction/motion contract test to reflect the four-frame reveal contract.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
scripts/perf/ui.mjs	Adds new phase sample buckets and report metrics for tail alignment timing boundaries.
e2e/remote-app.spec.ts	Instruments tail alignment timing in the cold-mount benchmark scenario and writes new phase outputs.
docs/PERFORMANCE.md	Documents tail-alignment phase and the four-consecutive-ready-frames reveal contract.
apps/web/test/interaction-motion.test.ts	Updates the contract test to assert the four-frame reveal stability constant.
apps/web/src/features/transcript/TranscriptTimeline.tsx	Changes reveal stability frame requirement from 8 to 4 with updated rationale comments.

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