feat(exporters): implement PNG image exporter with grid and legend rendering

- Added PngExporter struct to generate PNG images from voxel grids.
- Implemented image dimensions calculation, drawing primitives, and legend rendering.
- Integrated PNG export functionality into the CLI with a new --cell-size flag.
- Removed outdated design spec and implementation plan documents.

crates/exporters/src/litematica.rs

@@ -37,7 +37,7 @@ pub struct Metadata {
 #[serde(rename_all = "PascalCase")]
 pub struct Region {
     pub block_state_palette: Vec<BlockState>,
-    pub block_states: Vec<i64>,
+    pub block_states: fastnbt::LongArray,
     pub position: Vector3,
     pub size: Vector3,
 }
@@ -144,7 +144,7 @@ impl StructureExporter for LitematicaExporter {
             "TextRegion".to_string(),
             Region {
                 block_state_palette: palette_list,
-                block_states: packed_states,
+                block_states: fastnbt::LongArray::new(packed_states),
                 position: Vector3 { x: 0, y: 0, z: 0 },
                 size: dimensions,
             },