fix(render): use correct fallback row index in rerenderRows

[lukecotter]

Problem

rerenderRows falls back to the last row index when topRow === false, but writes
this.rows.length - 1. this.rows is a zero-arity method, so .length is 0 and
the expression is always -1.

Fix

Use this.rows().length - 1 — the last display-row index.

Performance

Neutral. 500k rows, K=5, medians (measured together with the follow-up filter-window
fix, see the dependent PR):

Metric	Before	After
initial render (ms)	97.6	103.2
initial render, variable heights (ms)	105.6	108.1
fling churn, uniform	14205	14205
fling churn, variable	3935	3935

[lukecotter]

Folded into #4931, which now carries this fix unchanged as its own commit, so the four VirtualDomVertical state-tracking fixes are one review instead of four. Closing to keep the series small — happy to split it back out if you would rather review them separately.