Take the dodge/burn mask tint off the canvas while a mask is adjusted

[Icodextrin]

What

The canvas tint that shows a Dodge & Burn mask's extent now mutes for the duration of an edit gesture. This also applies to any other masks that overlap with the mask being modified.

• Triggered by a press and hold on Burn, Feather or Grade.
• Grabbing a vertex does the same.
• Every mask whose tinted area intersects the one being worked on drops its fill too.
• Outlines and handles stay visible.

Why

The previous workflow required manually hiding the mask, tweaking the values, then revealing the mask if you needed to change the area again. This change makes dodge and burn tweaks more ergonomic.

How overlap is decided

overlapping_masks() (negpy/features/local/logic.py) rasterises each mask on a coarse grid in raw-normalised space, through the same rasterise() that the render, the canvas tint and the printing-notes map already share. Consequences worth noting:

• The awkward shapes come out right without special cases: a Card Edge covers its whole exposed side of the frame, an inverted mask covers its surround, and a polygon's area is its smoothed outline (which bulges roughly a tenth of an edge past the control points), not the vertex hull.
• Raw space makes the answer rotation-, zoom- and pan-invariant, since geometry moves every mask together — no screen coords or UV grid involved.

Reviewer notes

• The muted set is fixed when the gesture starts, not recomputed per frame. Recomputing would make a neighbour blink on and off as a vertex was dragged across it.
• A tint counts as present above alpha 0.25. Masks sharing a feather tail mute together; masks a clear gap apart do not. At the default feather that boundary falls between a gap of 0.05 and 0.20 of the frame. This threshold is the one knob if the behaviour reads as too eager or too shy.
• _try_start_vertex_edit was restructured to a single exit so the mute and the repaint happen once instead of being duplicated across its two branches. Behaviour is unchanged: the insert branch is polygon-only, so the oval-center anchor still resolves to None there.
• Nothing here touches either render engine — the tint is overlay paint only, so there is no CPU/GPU parity surface and no docs/PIPELINE.md change. docs/USER_GUIDE.md gains one bullet in the Dodge & Burn section.

Testing

• New tests/test_local_overlap.py covers the pure overlap function: symmetry, feather reach in both directions, card edge, invert, and unfinished/missing masks.
• tests/test_local_overlay.py rebuilt around three masks (selected / crossing / clear) and asserts through a real paint onto a QImage that the first two go to alpha 0 while the third keeps its tint — for both a slider drag and a vertex drag.
• tests/test_local_sidebar.py asserts all three sliders emit the drag signal on grab and release.
• make all clean: lint, types, 4285 passed / 11 skipped.