fix(web): keep compaction status semantic row singular

[lyc-aon]

Problem

During TranscriptTimeline's cold-mount handoff, the hidden LegendList and the paint-only warm-tail overlay coexist. Both copies of a working row exposed data-transcript-status, so the compacting status intermittently appeared twice in the DOM. The real headed reproduction failed 8/10 first attempts with a two-element strict-mode violation.

Fix

Mark cold-mount overlay rows as paint-only ghosts. Working-row copy and pixels stay unchanged, but only the live transcript row carries the lifecycle status hook. The overlay was already aria-hidden; no locator, timing, CSS, or assertion was weakened.

Proof

• focused web behavior test: 3/3 passed
• headed Chromium on isolated Xvfb, retries disabled: 10/10 consecutive focused attempts passed
• implementation-agent stress: an additional 20/20 consecutive focused attempts and full file 5/5 passed
• pnpm --filter @t4-code/web typecheck: passed

This is release-critical because the prior headed release gate was genuinely flaky.