Add rustfmt formatting policy and reformat codebase

Add rustfmt.toml with stable defaults and apply cargo fmt to all
source files. This establishes a consistent formatting baseline
for CI enforcement.

src/cache/coord_cache.rs

@@ -6,10 +6,10 @@
 
 use std::collections::HashMap;
 
-use super::entry::CacheEntry;
 use super::CacheStats;
-use crate::tree::TreeCoordinate;
+use super::entry::CacheEntry;
 use crate::NodeAddr;
+use crate::tree::TreeCoordinate;
 
 /// Default maximum entries in coordinate cache.
 pub const DEFAULT_COORD_CACHE_SIZE: usize = 50_000;