Add-ons: Color Effects — live Vector Halftone effect

Effect › Color Effects › Vector Halftone… (addon.color.halftone, also in
Object › Plug-ins › Color Effects) draws its object as the halftone dots
Object › Vector Halftone makes, live: mono (one dot colour, or each dot in
the art's colour under it) or CMYK screens, the five dot shapes,
frequency, angle, invert and clip to the art's outline.

- The screens, dots and caps move to vectorcraft_effects::addons::halftone,
  shared by the command (its behaviour and tests unchanged) and the effect.
- The renderer samples the art (vectorcraft_render::addons::halftone) and
  keeps the dots by the object's content and what it reads from the
  document, so unchanged art isn't sampled again; past 100000 cells per
  screen the object draws as it is.
- Expand Appearance turns the object into its dots, and the SVG, PDF, EPS
  and AI exporters write them as paths.
- Its dialog reuses the command's fields, without Keep Original.

Deferred: stochastic and radial patterns, a dot-gain curve, per-screen
separation settings, type or symbols as dots.

crates/engine/src/cmd/effectcmd.rs

@@ -69,7 +69,7 @@ pub fn specs() -> Vec<CommandSpec> {
             "Expand Appearance",
             ["Object"],
             None,
-            "{ids?|id?: [..], target?} turn each object's appearance into objects: every fill and stroke becomes an object of its own (a stroke stays a stroke, with its weight, dashes, arrowheads and width profile, on its own copy of the path, which object.expand or object.path.outlineStroke outline; brushed strokes become their brush art) grouped in paint order under the object's id and transparency, geometry effects are baked, raster effects become an embedded image (shadows and outer glows below the art; blur, feather and inner glow replace it; an object whose only raster effect is its own Rasterize becomes the image object.rasterize makes with the effect's options), type with effects or fills of its own is outlined; Crop Marks become a group of the object and its marks; a group's or layer's own fills and strokes become objects among its members, which are expanded too. An outlined frame of threaded text leaves its thread with the text it shows, and a frame kept as type in a group stays in its thread. Hidden fills, strokes and effects are dropped. Enabled when a selected or targeted object's appearance isn't basic (`ids` then pick which objects) → {ids, warning? (an image in the art couldn't be decoded, as object.rasterize says)}",
+            "{ids?|id?: [..], target?} turn each object's appearance into objects: every fill and stroke becomes an object of its own (a stroke stays a stroke, with its weight, dashes, arrowheads and width profile, on its own copy of the path, which object.expand or object.path.outlineStroke outline; brushed strokes become their brush art) grouped in paint order under the object's id and transparency, geometry effects are baked, raster effects become an embedded image (shadows and outer glows below the art; blur, feather and inner glow replace it; an object whose only raster effect is its own Rasterize becomes the image object.rasterize makes with the effect's options), type with effects or fills of its own is outlined; Crop Marks become a group of the object and its marks; Vector Halftone (addon.color.halftone) becomes a group of its dot paths, one per ink; a group's or layer's own fills and strokes become objects among its members, which are expanded too. An outlined frame of threaded text leaves its thread with the text it shows, and a frame kept as type in a group stays in its thread. Hidden fills, strokes and effects are dropped. Enabled when a selected or targeted object's appearance isn't basic (`ids` then pick which objects) → {ids, warning? (an image in the art couldn't be decoded, as object.rasterize says)}",
             can_expand,
             expand_appearance
         ),
@@ -646,6 +646,17 @@ fn expanded_node(d: &mut Document, mut n: Node, brushes: &[vectorcraft_brush::Br
         out.extend(expanded.into_iter().map(|(id, at)| (id, at + 1)));
         return Some(m);
     }
+    // Vector Halftone: the object becomes its dots, a group (with `id`) of paths with ids of their own.
+    if let Some(mut m) = vectorcraft_render::addons::halftone::art(d, &n) {
+        let mut seen = std::collections::HashSet::from([id]);
+        for c in m.children_mut().into_iter().flatten() {
+            effects::fresh_ids(d, Arc::make_mut(c), &mut seen);
+        }
+        if !listed_n {
+            out.push((id, 0));
+        }
+        return Some(m);
+    }
     let image = raster_image(d, &n);
     let mut v = n.clone();
     rasterfx::strip_raster(&mut v, false);