Module cache 

Cache module - provides file caching with hash-based change detection

This implementation follows the TypeScript reference:

• First read in a session: returns full content, caches it
• Second read (unchanged): returns “[unchanged, X lines, Y tokens saved]”
• File changed: returns diff showing what changed
• Multi-session: each session tracks independently

Structs

CacheStore

CacheStore provides file caching with SHA-256 hash-based change detection