Port selected-colour editing through main d42f9da94 and its engine/UI
module layout. Preserve native channel targets, drawing previews, tone
editors and filter behavior in the split modules, with their existing
regressions and public facade exports.
Integrate main 59cb4c49b, including egui 0.37, the rebuilt Histogram
panel, live Mixer Brush and smart-filter mask painting. Preserve selected
native components and captured mask/dialog targets through the merge,
with preview, replay, undo and mode/depth regressions.