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

cli/src/cmds/export.rs

@@ -1,64 +0,0 @@
-use clap::Args;
-use std::path::Path;
-
-use crate::util::output::{self, OutputFormat};
-use crate::util::{load_world, progress};
-
-#[derive(Args)]
-pub struct ExportArgs {
-    /// Path to Terrafier world directory
-    pub world: String,
-    /// Output directory for Minecraft save
-    #[arg(short, long)]
-    pub output: String,
-    /// Minecraft data version (default: 3954 for 1.21)
-    #[arg(long, default_value = "3954")]
-    pub data_version: i32,
-}
-
-pub fn cmd_export(
-    args: ExportArgs,
-    format: &OutputFormat,
-    dry_run: bool,
-    validate: bool,
-) -> anyhow::Result<()> {
-    let world_path = Path::new(&args.world);
-    log::info!("Exporting world from: {}", world_path.display());
-
-    let world = load_world(world_path)?;
-    terrafier_core::io::export::validate_export(&world)?;
-
-    let tile_count: usize = world.dimensions.iter().map(|d| d.tiles.len()).sum();
-
-    if validate || dry_run {
-        output::print_result(
-            format,
-            &serde_json::json!({
-                "status": "validated",
-                "input": args.world,
-                "output": args.output,
-                "tiles": tile_count,
-            }),
-        );
-        if dry_run {
-            return Ok(());
-        }
-    }
-
-    let bar = progress::new_bar(tile_count as u64, "Exporting...");
-    terrafier_core::io::export::export_to_save(&world, Path::new(&args.output))?;
-    progress::finish_with("Export complete", bar);
-
-    log::info!("World exported to: {}", args.output);
-    output::print_result(
-        format,
-        &serde_json::json!({
-            "status": "exported",
-            "input": args.world,
-            "output": args.output,
-            "tiles": tile_count,
-        }),
-    );
-
-    Ok(())
-}