Fix #122: Object > Vector Halftone

The issue asked for a vector halftone that turns art or an image into
vector dots, lines or shapes by tone, with angle, frequency, shape, mono
or CMYK separations and clipping to the object. Nothing like it existed.

object.vectorHalftone renders the selection offscreen, samples its tone
once per cell of a grid turned by the screen angle and places a shape
whose area follows the tone (circle, ellipse, square, diamond or line).
Mono screens follow lightness in one colour; CMYK screens follow the ink
amounts the CMYK output separates, one screen per ink at the classic
angles, multiplied over each other. Dots are clipped to the art's
outline as real geometry (only the dots crossing it go through a
boolean). It is a command rather than a live effect: the live pipeline
can't sample rendered tone when exporting. Its dialog previews live and
OK keeps it as one undo step.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docs/control-protocol.md

@@ -440,4 +440,7 @@ agent can call directly. Mirror & Cut's tool options (`tool.setOption`, shown in
 Colour adjustment effects (Effect → Color Adjustments) open the `effect` dialog like the other effects
 (`effect.dialog {effect: "adjust.hueSaturation"}`): its fields are the effect's parameters (sliders for the amounts,
 `channel` for Curves and Levels, `points` for Curves, `color` for Shift to Color) and they preview live; set them with
-`ui.dialog.set` and `ui.dialog.confirm` to apply (Curves also has a graph: drag its points).
+`ui.dialog.set` and `ui.dialog.confirm` to apply (Curves also has a graph: drag its points). Object → Vector
+Halftone… (`engine.execute {command: "ui.menuDialog", params: {command: "object.vectorHalftone"}}`) opens the
+`vectorHalftone` dialog (fields `shape`, `frequency`, `angle`, `mode`, `color`, `invert`, `clip`, `keepOriginal`,
+`preview`): it previews live and `ui.dialog.confirm` keeps the halftone as one undo step.