- Add early return for len==0 (Ok(&[])) matching std::io semantics
- Add slot.len > 0 guard to cache hit predicate to prevent empty-slot
false matches
- Replace unchecked arithmetic with checked_add/saturating_add for
request_end, block_end, and post-read coverage check
- Fix misleading comment about get(file,0,0) behavior on miss path
- Strengthen clear() to fully reset block_offset and last_access
- Register read_cache module in io/mod.rs
- Add 4 regression tests: zero-len on fresh/populated cache,
zero-len at u64::MAX, overflow error on nonzero read at u64::MAX
