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).
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