Record intersection as built in the requirement and the mask plan

FR-DEV-19a said a part is added to the mask or taken out of it, and
mask-editing.md listed Intersect as an M2 item with nothing built. Both
now say what shipped: the requirement names the third join and why it is
a product rather than a minimum, and how old and new sidecars read across
it; the plan marks the blend-table row built, names the tests that hold
the GPU to the definition, and splits M2 into what is done and what is
still outstanding (joining non-painted parts from the panel, per-part
distance fields, folding two layers).

docs/dev/requirements.md

@@ -544,9 +544,17 @@ decade; a mask that cannot be corrected is the reason an edit leaves this applic
 editor.
 
 **FR-DEV-19a — Mask composition.** A layer's mask is an ordered list of parts, each naming a source
-and how it joins the mask before it — added to it, or taken out of it. A layer of one part is
-exactly the layer that existed before this, and reads and writes the same sidecar. The parts of a
-layer merge under FR-NC-9 as the layer does, and each carries its own edge treatment.
+and how it joins the mask before it — added to it, taken out of it, or intersected with it, keeping
+only where both agree. A layer of one part is exactly the layer that existed before this, and reads
+and writes the same sidecar. The parts of a layer merge under FR-NC-9 as the layer does, and each
+carries its own edge treatment.
+
+*Intersection added 2026-09-24* (issue #9, `mask-editing.md` M2). Union and subtraction were built
+first; the selections that most need composing — the sky that is also bright, the subject that is
+also skin — are intersections, and spelling one as a subtraction needs a part that selects the
+complement. Intersection is the product of the two coverages, which is the minimum wherever either
+side is fully in or out. A sidecar written before it never names it; a build from before it reads
+the word as a union, which keeps the part visible rather than dropping it.
 
 **FR-DEV-19b — Hand correction.** A part may be painted, with add and erase strokes, at a radius,
 hardness and flow the photographer sets. Strokes are stored as normalised source coordinates and