The auto masks arrive in a second and cannot then be changed by a pixel: no
brush, no way to cut one selection out of another, no way to drag a boundary
that stopped inside a shoulder, and no way to see the alpha a layer actually
produces — the canvas overlay draws what the model detected, not the mask.
docs/mask-editing.md is how that closes. A layer stops holding one source and
holds an ordered list of parts, each naming how it joins the mask before it,
so painting on an auto mask, subtracting, and intersecting a subject with a
luminance band are all one mechanism. Notes what the tree already has (the
whole brush is written and reachable from no control), what the set operations
actually cost (three blend states, no new texture), why the edge push is a
warp rather than a local morphology, and why painting has to draw
incrementally. Ends with the four decisions the build needs first.
