Oval and card-edge dodge/burn masks, plus per-mask invert

[marcinz606]

Dodge & Burn had one shape: a clicked polygon. Two common darkroom moves did not fit it.

• Burning through a hole in the card (or dodging with a wand). A smooth oval took a dozen clicked vertices and was awkward to reshape.
• The card-edge burn — the graduated sky burn a printer makes by moving a card across the paper. It needs an unbounded ramp, which a closed polygon and a Gaussian cannot express.

Two more gaps came out of the same model: no way to act on everything except a shape, and smooth_polyline rounds every corner.

What changed

PolygonMask becomes LocalMask and gains shape and invert. The vertices stay the universal store and shape says how to read them:

Shape	Vertices	Outline
Polygon	N control points	Catmull-Rom, closed (unchanged)
Oval	3: centre, one end of each axis	the unit circle under the matrix [u v], 64 samples
Gradient	2: full exposure, then zero	none; a smoothstep ramp along the axis

Storing an oval as points, not as a centre plus radii plus an angle, keeps geometry mapping shape-blind: every control point still goes through map_coords_to_geometry, so rotation, flips and distortion follow as they did. The points map to pixels before the outline is generated, so the raw-image aspect needs no special case. The axes need not be perpendicular, which makes a tilted oval the same expression as a round one.

An oval outline is generated, so the existing fillPoly and Gaussian feather serve it with no new fill path. A gradient ignores Feather, because the distance between its two handles is the softness. invert is 1 - alpha after the feather.

One function (local/logic.rasterise) now serves the render, the canvas tint and the printing-notes map, so the three cannot describe different shapes. The GPU consumes the same CPU-rasterised map, so there is no shader work and no parity surface.

Canvas

Both new tools drag out their shape, and one local_mask_created(shape, points) signal replaces lasso_completed. Outlines and control points are now tracked separately: the old code hit-tested the control polygon, which would have hit-tested an oval's triangle. A card edge draws a solid line at full exposure and a dashed line at zero, and hits near its axis. The oval centre handle moves the axes with it. Midpoint + and vertex delete stay polygon-only, because only a polygon can change its point count.

UI

Three tool buttons, an Invert toggle, a shape icon per list row, and Feather disabled on a card edge. Two shortcut-registry entries with empty default keys — no free key was worth a collision.

Compatibility

shape and invert default in _build_local, so a mask saved before this change loads as a polygon. Nothing goes through migrations.py.

Verification

• make all green: 3499 passed, 2 skipped.
• Headless end-to-end on a real scan, on both engines: oval burn +1 st gave −0.0495, card-edge dodge −1 st gave +0.0757, and a region outside both masks moved ±0.0000. GPU against CPU: max diff 0.0078 (2 codes at 8 bit), mean 2e-7.

Left out

A freehand brush mask, a hard-edge toggle, and tonal-range gating. The first two fit the same shape slot. The third needs the normalised density threaded into compute_local_maps, which the GPU rasterises per tile before the print stage, so it is separate work.