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

Cargo.lock

@@ -1110,6 +1110,7 @@ dependencies = [
  "objc2-foundation 0.2.2",
  "parking_lot",
  "percent-encoding",
+ "pollster",
  "profiling",
  "raw-window-handle",
  "static_assertions",
@@ -1117,6 +1118,7 @@ dependencies = [
  "wasm-bindgen-futures",
  "web-sys",
  "web-time",
+ "wgpu",
  "winapi",
  "windows-sys 0.59.0",
  "winit",
@@ -1661,6 +1663,18 @@ dependencies = [
  "bitflags 2.11.1",
 ]
 
+[[package]]
+name = "gpu-allocator"
+version = "0.27.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "c151a2a5ef800297b4e79efa4f4bec035c5f51d5ae587287c9b952bdf734cacd"
+dependencies = [
+ "log",
+ "presser",
+ "thiserror 1.0.69",
+ "windows",
+]
+
 [[package]]
 name = "gpu-descriptor"
 version = "0.3.2"
@@ -2831,6 +2845,12 @@ dependencies = [
  "zerocopy",
 ]
 
+[[package]]
+name = "presser"
+version = "0.3.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "e8cf8e6a8aa66ce33f63993ffc4ea4271eb5b0530a9002db8455ea6050c77bfa"
+
 [[package]]
 name = "prettyplease"
 version = "0.2.37"
@@ -3026,6 +3046,12 @@ dependencies = [
  "rand_core 0.5.1",
 ]
 
+[[package]]
+name = "range-alloc"
+version = "0.1.5"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "ca45419789ae5a7899559e9512e58ca889e41f04f1f2445e9f4b290ceccd1d08"
+
 [[package]]
 name = "raw-window-handle"
 version = "0.6.2"
@@ -3578,9 +3604,11 @@ dependencies = [
  "serde",
  "serde-big-array",
  "tempfile",
+ "terrafier-biome-db",
  "terrafier-fastanvil",
  "terrafier-nbt",
  "terrafier-noise",
+ "terrafier-palette-compress",
  "thiserror 2.0.18",
 ]
 
@@ -3601,10 +3629,12 @@ dependencies = [
  "anyhow",
  "eframe",
  "egui",
+ "egui-wgpu",
  "env_logger",
  "log",
  "rfd",
  "terrafier-core",
+ "wgpu",
 ]
 
 [[package]]
@@ -3810,9 +3840,9 @@ dependencies = [
 
 [[package]]
 name = "typenum"
-version = "1.20.0"
+version = "1.20.1"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "40ce102ab67701b8526c123c1bab5cbe42d7040ccfd0f64af1a385808d2f43de"
+checksum = "b6f5e870be6c3b371b77fe0ee0bafb859fa4964b4404c27de1d380043c4dda20"
 
 [[package]]
 name = "uds_windows"
@@ -4221,6 +4251,7 @@ dependencies = [
  "document-features",
  "js-sys",
  "log",
+ "naga",
  "parking_lot",
  "profiling",
  "raw-window-handle",
@@ -4268,13 +4299,16 @@ dependencies = [
  "android_system_properties",
  "arrayvec",
  "ash",
+ "bit-set",
  "bitflags 2.11.1",
+ "block",
  "bytemuck",
  "cfg_aliases",
  "core-graphics-types",
  "glow",
  "glutin_wgl_sys",
  "gpu-alloc",
+ "gpu-allocator",
  "gpu-descriptor",
  "js-sys",
  "khronos-egl",
@@ -4289,6 +4323,7 @@ dependencies = [
  "ordered-float",
  "parking_lot",
  "profiling",
+ "range-alloc",
  "raw-window-handle",
  "renderdoc-sys",
  "rustc-hash 1.1.0",
@@ -4298,6 +4333,7 @@ dependencies = [
  "web-sys",
  "wgpu-types",
  "windows",
+ "windows-core",
 ]
 
 [[package]]