d37ed6df68
- 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