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

core/src/ops/layers/terrain/biome.rs

@@ -0,0 +1,59 @@
+use crate::coords::TILE_SIZE;
+use crate::model::layers::{DataSize, LAYER_BIOME};
+use crate::model::tile::Tile;
+use crate::ops::layers::LayerGenerator;
+
+pub struct BiomeGenerator;
+
+impl LayerGenerator for BiomeGenerator {
+    fn layer_id(&self) -> u32 {
+        LAYER_BIOME
+    }
+
+    fn generate(&self, tile: &mut Tile, _seed: u64) {
+        let data = from_terrain(&tile.terrain);
+        let buf = tile.ensure_layer(LAYER_BIOME, DataSize::Byte);
+        if let crate::model::tile::LayerBuffer::Byte(bytes) = buf {
+            bytes.copy_from_slice(&data);
+        }
+    }
+}
+
+/// Map terrain types to biome indices used by BiomeLayerExport.
+/// Biome index → MC biome ID mapping (from biome_res.rs):
+///   0 → 1 (Plains),  1 → 2 (Desert),  2 → 4 (Forest),
+///   3 → 5 (Taiga),   4 → 6 (Swamp),   8 → 0 (Ocean)
+const TERRAIN_TO_BIOME: [u8; 7] = [
+    0, // Desert → Plains (index 0)
+    0, // Grass  → Plains (index 0)
+    2, // Forest → Forest (index 2)
+    3, // Rock   → Taiga  (index 3)
+    1, // Sand   → Desert (index 1)
+    4, // Swamp  → Swamp  (index 4)
+    8, // Water  → Ocean  (index 8)
+];
+
+pub fn from_terrain(terrain: &[u8]) -> Vec<u8> {
+    terrain
+        .iter()
+        .map(|&t| {
+            let idx = t as usize;
+            if idx < TERRAIN_TO_BIOME.len() {
+                TERRAIN_TO_BIOME[idx]
+            } else {
+                0
+            }
+        })
+        .collect()
+}
+
+#[allow(dead_code)]
+pub fn biome_at(tile: &Tile, local_x: usize, local_z: usize) -> u8 {
+    let idx = local_z * TILE_SIZE + local_x;
+    let terrain = tile.terrain[idx] as usize;
+    if terrain < TERRAIN_TO_BIOME.len() {
+        TERRAIN_TO_BIOME[terrain]
+    } else {
+        0
+    }
+}