A QPixmap snapshot cannot follow palette changes, so an authored `currentColor`
glyph would freeze at whatever color it was first rendered with. Add a QIcon
engine that resolves the sentinel from the current palette on every pixmap
request, caches by resolved path/color/size/DPR, and falls back to Qt's normal
loader for authored multi-color art and raster overrides.
Purely additive. No behavior change yet.
