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>

docs/mcp.md

@@ -259,3 +259,18 @@ it by multiplying. Older files that listed Overprint Black objects in the docume
 {"name":"run_command","arguments":{"command":"attributes.info","params":{"ids":[12]}}}
 {"name":"run_command","arguments":{"command":"view.overprintPreview","params":{"on":true}}}
 ```
+
+## Edit Colors and Recolor Artwork
+
+`edit.colors.invert`, `edit.colors.toCMYK`, `edit.colors.toGrayscale`, `edit.colors.toRGB`, `edit.colors.saturate`,
+`edit.colors.adjustBalance` and `recolor.apply` recolour everything inside the selection (or `ids`) in one undo step:
+fills, strokes, text, gradient stops, gradient-mesh points, embedded images (a recoloured copy of the image; linked
+images are left alone) and the tiles of pattern fills and strokes (a recoloured copy saved as a new pattern swatch,
+e.g. "Dots 2"; the original pattern is untouched). `includeImages: false` / `includePatterns: false` leave images or
+patterns out. The Blend commands also grade gradient meshes, which keep their shading.
+
+```json
+{"name":"run_command","arguments":{"command":"recolor.colors","params":{}}}
+{"name":"run_command","arguments":{"command":"recolor.apply","params":{"map":{"#ff0000":"#0055ff"},"includeImages":false}}}
+{"name":"run_command","arguments":{"command":"edit.colors.adjustBalance","params":{"mode":"cmyk","m":-20,"k":10}}}
+```