Fix heatmap rendering failure for long segments

The heatmap uses site:O (ordinal) with width=Step(16px). With no initial
brush, Vega-Lite renders ALL sites at once — fine for short segments
(~300 sites) but catastrophic for long ones like PB2 (~2280 sites =
36 000+ px wide). Fix by setting an initial brush value covering the
first 200 sites so the heatmap starts at a manageable width.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: Haddox