Refactor structure, implement layers, add Erode/Fill/layer panel

- Restructure folder hierarchy: max 4 .rs per dir using subfolders
  (ops/operations -> brush/ + shaping/, model/ -> world/, io/* -> subtypes, etc.)
- Fix golden_test.rs comment, extract BrushApply helper, add World::with_heightmap
- Implement 6 layer data generators (Caves, River, Frost, Trees, Biome, Resources)
  with noise-based (OpenSimplex) and terrain-derived algorithms
- Fix clippy warnings, dead code cleanup, import path fixes
- GUI: add Erode&Fill tool modes with radio buttons/sliders/apply_tool
- GUI: add Layers panel with 6 checkboxes + CPU viewport overlay rendering

gui/src/main.rs

@@ -1,15 +1,15 @@
-fn main() -> Result<(), Box<dyn std::error::Error>> {
+fn main() -> Result<(), eframe::Error> {
     env_logger::init();
-    let native_options = eframe::NativeOptions::default();
+
+    let native_options = eframe::NativeOptions {
+        renderer: eframe::Renderer::Wgpu,
+        viewport: egui::ViewportBuilder::default().with_inner_size([1280.0, 800.0]),
+        ..Default::default()
+    };
+
     eframe::run_native(
         "Terrafier",
         native_options,
-        Box::new(|_cc| Ok(Box::new(app::TerrafierApp::new()))),
-    )?;
-    Ok(())
+        Box::new(|cc| Ok(Box::new(terrafier_gui::TerrafierApp::new(cc)))),
+    )
 }
-
-mod app;
-mod dialogs;
-mod tools;
-mod view;