M3.55: Edit Colors and Recolor Artwork also recolour gradient meshes, embedded images and pattern fills

One shared colour visitor (`colorcmds::Recolor`, used by Edit Colors, Recolor Artwork, the Blend
commands, the proxy Invert/Complement and Document Color Mode) maps solid colours, gradient stops
and text runs as before, and now also:
- gradient-mesh points (with fills); Blend grades meshes by their average and keeps their shading;
- embedded images' pixels (`includeImages?`, default on): the image becomes a recoloured copy under
  a content key, the original stays for undo and other objects; linked images are left alone;
- the tiles of pattern fills and strokes (`includePatterns?`, default on): one recoloured copy per
  pattern saved as a new pattern swatch ("Dots 2"), the original untouched.
Everything stays one undo step; only nodes that change are copied, in one walk of the tree.

- doc: `ImageBlob::map_rgb` (decode, map each distinct colour once, keep alpha, re-encode PNG),
  `ImageBlob::content_key` (now also used by image import) and `ImageBlob::png`.
- `recolor.colors` lists mesh points and pattern tile colours too.
- patterncmds: `add_pattern` adds a pattern and its swatch (shared by Make Pattern and Save a Copy).

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

crates/engine/src/cmd/docmenu.rs

@@ -8,7 +8,7 @@ use std::sync::Arc;
 use serde_json::{Value, json};
 use vectorcraft_doc::{CharStyle, ColorMode, Guide, Node, NodeId, NodeKind, TextKind};
 
-use super::colorcmds::{map_node_colors, to_cmyk};
+use super::colorcmds::{Recolor, Scope, to_cmyk};
 use super::edit::selected_roots;
 use super::*;
 
@@ -466,9 +466,8 @@ fn color_mode(s: &mut Session, p: &Value) -> Result<Value> {
     s.edit("Document Color Mode", |d, _| {
         d.color_mode = mode;
         if convert {
-            for l in &mut d.layers {
-                map_node_colors(Arc::make_mut(l), &f, true, true);
-            }
+            let layers: Vec<NodeId> = d.layers.iter().map(|l| l.id).collect();
+            Recolor::new(Scope { fill: true, stroke: true, images: false, patterns: false }, &f).run(d, &layers);
             for sw in d.swatches_iter_mut() {
                 if let vectorcraft_color::Paint::Solid { color, .. } = &mut sw.paint {
                     *color = f(*color);