feat: add watercolor brush category with presets and splat renderer

- Implemented a new crate `hcie-watercolor-brushes` containing watercolor brush presets and a procedural splat preview renderer.
- Integrated watercolor brushes into the iced GUI's Brushes panel, adding a new category for watercolor brushes.
- Updated the `BrushCategory` enum to include `Watercolor` and modified the rendering logic to display watercolor presets.
- Added functionality to generate watercolor splat thumbnails using a noise-based algorithm.
- Ensured that existing brush styles and engine crates remain unmodified, adhering to project constraints.

Cargo.lock

@@ -2756,6 +2756,7 @@ dependencies = [
  "futures-util",
  "hcie-build-info",
  "hcie-engine-api",
+ "hcie-watercolor-brushes",
  "iced",
  "iced-panel-adapter",
  "image 0.25.10",
@@ -2921,6 +2922,13 @@ dependencies = [
  "tokio",
 ]
 
+[[package]]
+name = "hcie-watercolor-brushes"
+version = "0.1.0"
+dependencies = [
+ "hcie-engine-api",
+]
+
 [[package]]
 name = "heck"
 version = "0.4.1"