refactor: split magic_numbers into per-module files

Extracted all doc-commented magic numbers into module-local magic_numbers.rs
files, co-located with the module that owns them. No logic changes.

- entities/shared_systems/magic_numbers: pathfinding + dig constants
- world/generation/terrain/magic_numbers: terrain + cave constants
- world/generation/forestry/magic_numbers: tree constants
- world/tiles/magic_numbers: A*, benchmark, memory constants
- entities/item/magic_numbers: item sprite z-offset
- Removed src/magic_numbers.rs (central file deleted)

Constants imported as `crate::{module}::magic_numbers::*` from each
consumer. src/constants.rs retains only structural constants (PIXEL_RATIO,
TILE_SIZE, SEED, pathfinding tier thresholds).

src/world/tiles/tilemap.rs

@@ -35,6 +35,7 @@ use super::chunk_data::ChunkData;
 use crate::constants::ITILE_SIZE;
 use crate::entities::item::drop_table::DropTable;
 use crate::world::chunks::{world_to_chunk, CHUNK_SIZE, Z_ABOVE, Z_BELOW};
+use crate::world::tiles::magic_numbers::ASTAR_DEFAULT_WEIGHT;
 
 /// Packed floor tile data for efficient storage. ~35 bytes vs 76 bytes tuple.
 #[derive(Clone, Debug)]
@@ -296,7 +297,7 @@ impl TileMap {
         self.floor_tiles
             .get(&world_pos)
             .map(|t| t.astar_weight)
-            .unwrap_or(100)
+            .unwrap_or(ASTAR_DEFAULT_WEIGHT)
     }
 
     /// Remove a fixture tile, clearing both the HashMap entry and ChunkData bitsets.