Splitter strips widen to border +/- 1 help-padded column so the thin
border is easy to target; mousedown records the cursor's offset from
the border column and drag subtracts it, so the border tracks the
cursor instead of jumping. Padded columns must never overlap
interactive content (rect-based hit grid). Dead one-off render
harness scripts/_hv.ts removed. Tests cover far/inner-edge grabs
and no-resize on padded-column clicks.
