fix(render): clamp vDomTopPad to zero when removing top rows

[lukecotter]

Problem

In _removeTopRow, accumulated actual row heights can push vDomTopPad slightly
negative. That sets a negative paddingTop and lets rendered content drift above the
viewport.

Fix

Clamp vDomTopPad to zero after the adjustment. One line.

A note on test coverage

An e2e test written for this passed with and without the fix, so it was deleted
rather than kept as misleading coverage. The failure is a sub-pixel drift that the
existing harness cannot resolve; the reasoning is in the diff comment instead.

Performance

Neutral. 500k rows, K=5, medians:

Metric	Before	After
initial render (ms)	97.6	102.0
initial render, variable heights (ms)	105.6	102.9
fling churn, uniform	14205	14205
fling churn, variable	3935	3935