perf: paint warm transcript before list mount

[wolfiesch]

Summary

• paint the existing exact warm transcript tail before mounting the hidden virtual list
• mount the measured list one animation frame later behind the visible warm copy
• preserve the existing tail-alignment and four-ready-frame reveal gates
• update the interaction contract, performance documentation, and provenance checksum

Matched VPS evidence

Seven runs per side at a 390x844 viewport with the frozen history-10k-v1 fixture:

Renderer-clock metric	Before	After	Change
Click to transcript shell	521.1 ms	457.1 ms	-12.3%
Click to tail aligned	590.6 ms	593.5 ms	+0.5%
Click to real list	720.9 ms	709.8 ms	-1.5%
Click to tail painted	776.9 ms	738.1 ms	-5.0%
Whole mount	2914.1 ms	2859.4 ms	-1.9%

The pre-click navigation DCL median moved from 86.1 ms to 95.5 ms. That phase occurs before the changed click path and is treated as shared-VPS noise rather than a product claim.

Verification

• provenance check: 5 manifests, 0 failures
• pnpm check
• interaction/motion tests: 22/22
• production web build
• focused 10k-history Playwright test on the VPS

[chatgpt-codex-connector]

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[Copilot]

Pull request overview

This PR improves transcript cold-mount perceived performance by ensuring the warm tail overlay paints before the virtualized LegendList mounts, while keeping the existing tail-alignment and stability-frame reveal gates intact.

Changes:

• Delay mounting LegendList by one animation frame so the warm tail overlay can paint first (TranscriptTimeline.tsx).
• Update the interaction/motion contract test to assert the new mount gating behavior (interaction-motion.test.ts).
• Refresh performance documentation to describe the updated paint/mount sequence and update provenance checksum (PERFORMANCE.md, provenance JSON).

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 1 comment.

File	Description
provenance/t3code/imports/f2-transcript-20260711.json	Updates provenance checksum for the modified transcript timeline implementation.
docs/PERFORMANCE.md	Documents the new warm-tail-first commit and deferred list mount behavior in UI perf metrics.
apps/web/test/interaction-motion.test.ts	Extends the interaction contract test to assert LegendList is mounted only after listMounted flips via rAF.
apps/web/src/features/transcript/TranscriptTimeline.tsx	Introduces listMounted to mount LegendList one frame later while preserving the cold-mount overlay reveal gating logic.

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