Fix WebGL texture atlas background cache churn on per-cell background colors

[s-celles]

Fixes #6074.

Problem

The WebGL glyph atlas is keyed on the full bg value, colour bits included
(cacheMap.get(key, bg, fg, ext)). But _drawToCache only reads the
background colour in three cases — _getBackgroundColor (unless
allowTransparency), _getMinimumContrastColor (unless minimumContrastRatio
is 1), and the inverse swap. When none apply, every background colour rasterizes
a pixel-identical tile, yet each gets its own atlas entry. Content that varies
the background per cell (animated backgrounds, heatmaps, diffs, ANSI art) then
misses on every cell of every frame and grows the never-evicted atlas without
bound.

Fix

Introduce a cacheKeyBg that drops the background colour bits when they cannot
change the tile, keeping the BgFlags that do (ITALIC, DIM, OVERLINE).
INVERSE (read from fg) stays in the key, since the inverse swap depends on
the colour. Same glyph over different background colours now reuses one atlas
entry.

Testing

• CharAtlasUtils.test.ts — unit coverage for cacheKeyBg (colour dropped;
  rasterizing flags and the transparency / contrast / inverse exceptions kept).
• WebglAtlasBackgroundChurn.test.ts — regression test that a per-cell
  background-colour sweep no longer multiplies atlas entries.

Assisted-by: AI