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.

hcie-iced-app/crates/hcie-iced-gui/Cargo.toml

@@ -18,6 +18,7 @@ path = "src/main.rs"
 [dependencies]
 hcie-build-info = { workspace = true }
 hcie-engine-api = { path = "../../../hcie-engine-api" }
+hcie-watercolor-brushes = { path = "../hcie-watercolor-brushes" }
 iced-panel-adapter = { path = "../iced-panel-adapter" }
 iced = { workspace = true }
 env_logger = { workspace = true }