feat(core): add disk index cache infrastructure

Add xxhash-rust and bincode workspace dependencies for fast hashing and serialization. Implement cache_util for cache directory/path resolution with versioning, and IndexCache for saving/loading line indices to disk with file-hash validation.

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-openagent)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

crates/core/Cargo.toml

@@ -15,6 +15,8 @@ regex.workspace = true
 memchr.workspace = true
 memmap2.workspace = true
 directories.workspace = true
+xxhash-rust.workspace = true
+bincode.workspace = true
 
 [dev-dependencies]
 insta.workspace = true