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fix(virtual-list): hide estimate-miss corrections while scrolling backlog (#418)

* test(virtual-list): add failing spec for estimate-miss corrections while scrolling backlog (#413)

Fixture: 300 items with heights cycling 64/96/128/512/700px, so the
average estimate converges to the cycle mean (~300px) while every
individual unmeasured item is wrong by up to 400px — variable heights
mean the misses cannot self-heal. The page carries a what/why/how
description and live red/green stats, and drives its own backlog sweep:
jump to the bottom, scroll up 250px per step through never-measured
items, tracking EVERY rendered item across frames (batched corrections
can cancel for any single item while its neighbors rearrange).

Pre-fix the sweep reads jumps=3 maxJumpPx=11281 totalJumpPx=14759 over
39 samples: corrections pile up behind the 100ms measurement debounce
(each scroll step resets it) and land as multi-thousand-pixel painted
lurches. The spec asserts on the fixture's own stat values and expects
zeros.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

* test(virtual-list): make the issue-413 fixture readable at a glance and immune to vacuous greens (#413)

Three hardenings driven by watching a human read the page:

- Re-run remounts the list ({#key runId}) so the backlog is unmeasured
  again — a second sweep over already-measured items produced zero
  corrections and read as a false green.
- Blank-coverage detection: after every sweep step and across five
  post-sweep settle samples (the debounced correction chain flushes in
  waves; a single sample races it), rendered items must fully cover the
  viewport. Reported as its own red/green row. Passes pre-fix — it is a
  tripwire so the fix cannot trade jump-hiding for unpainted frames.
- Visual honesty: a 700px item's textless body was indistinguishable
  from a broken blank screen. Item labels are now vertically centered
  with visible card borders, and the content layer behind the items is
  re... (continued)

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