Add setCellColorOverrides/clearCellColorOverrides to the terminal API,
backed by a per-line sparse override store consumed by the DOM renderer's
row factory. Covered cells render their original text in the given color,
with override equality participating in span merging so runs break only at
color boundaries. Overrides are keyed by absolute buffer line of the active
buffer; refresh is limited to the affected viewport rows.
This gives embedders a cheap way to recolor terminal text (e.g. keyword
highlighting) without decoration objects, which fragment text spans and
carry per-cell lookup cost in the DOM renderer, and which are force-hidden
while the alternate buffer is active.
ys-ll
d07813da37