feat(core): add Mmap error variant and memmap2 dependency

Add CoreError::Mmap(String) error variant for mmap operations and memmap2 = "0.9" as workspace dependency.

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

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

crates/core/Cargo.toml

@@ -13,6 +13,7 @@ parking_lot.workspace = true
 crossbeam-channel.workspace = true
 regex.workspace = true
 memchr.workspace = true
+memmap2.workspace = true
 directories.workspace = true
 
 [dev-dependencies]