Fix critical structural issues: undo/redo, surface import, brush strength, i18n macro, level.dat export, GPU fallback, Nether/End support, dead code removal, dim serde, CPU perf, wasm-host, GPU constants, clippy warnings

- Replace OnceLock with Mutex<Option> for snapshot system in all 9 operations (undo/redo now works after redo)
- Fix import surface detection: scan actual packed block data for highest solid block instead of always using +15
- Apply brush strength threshold (< 0.3) in PaintOperation for natural edge falloff
- Remove broken t!() macro variant with variable substitution
- Complete level.dat with RandomSeed, GameType, Difficulty, Spawn, Time, DataPacks, etc.
- GPU viewport falls back to CPU renderer for multi-tile worlds
- Export supports Nether (DIM-1/region) and End (DIM1/region) dimension subdirectories
- Remove dead FloodOperation (duplicate of PaintOperation)
- Add log::warn for malformed dimension tile keys in serde
- Reduce CPU viewport base_size from 64 to 32 (4x fewer pixels)
- Simplify wasm-host host_get_seed to direct cast
- Add named constants for GPU buffer sizes
- Fix unused import warning in cli/main.rs
- cargo fmt

gui/src/renderer/pipeline.rs

@@ -1,5 +1,10 @@
 use std::sync::Arc;
 
+const TILE_SIZE: u32 = 128;
+const TILE_CELLS: u32 = TILE_SIZE * TILE_SIZE;
+const HEIGHTMAP_PACKED_SIZE: u64 = (TILE_CELLS / 2) as u64 * 4;
+const TERRAIN_PACKED_SIZE: u64 = (TILE_CELLS / 4) as u64 * 4;
+
 #[repr(C)]
 #[derive(Clone, Copy)]
 pub struct Uniforms {
@@ -111,13 +116,13 @@ impl GpuRenderer {
         });
         let heightmap_buffer = device.create_buffer(&wgpu::BufferDescriptor {
             label: Some("heightmap_buffer"),
-            size: 8192 * 4,
+            size: HEIGHTMAP_PACKED_SIZE,
             usage: wgpu::BufferUsages::STORAGE | wgpu::BufferUsages::COPY_DST,
             mapped_at_creation: false,
         });
         let terrain_buffer = device.create_buffer(&wgpu::BufferDescriptor {
             label: Some("terrain_buffer"),
-            size: 4096 * 4,
+            size: TERRAIN_PACKED_SIZE,
             usage: wgpu::BufferUsages::STORAGE | wgpu::BufferUsages::COPY_DST,
             mapped_at_creation: false,
         });