EPIC: Zero-copy mmap read path for the File backend (large-object read RSS)

[27Bslash6]

Summary

The "zero-copy memory-mapped reads" advertised for compression=none do not exist: there is no pa.memory_map anywhere, the File backend os.reads the whole file then slices (two full heap copies), and the CK frame unwrap copies again. Setting compression=none today only inflates the payload with no read-RSS benefit. This epic builds the real path (measured target ~0.32x read RSS).

Sequenced sub-tasks

1. SerializationWrapper.unwrap copies the full payload on every read (incl. every L1 hit) #162 — SerializationWrapper.unwrap returns a memoryview (prerequisite; standalone win)
2. File backend mmap read — FileBackend.get() returns a buffer backed by pa.memory_map/mmap + memoryview slice past the 14-byte header, instead of os.read+slice. backends/file/backend.py:134,167
3. Lazy / per-batch checksum — the eager full-body xxHash3 (arrow_serializer.py:279) faults every page and erases the mmap RSS win; make it optional/streaming for the mmap path
4. compression=none default for the File backend (plaintext-scoped) while wire backends keep zstd — wire the coupling at create_cache_wrapper (decorators/wrapper.py:375 where config.backend is known) and extend the name-keyed serializer cache key (serializers/__init__.py:96) so File and Redis decorators don't share one zstd instance
5. Raise File max_value_mb (currently rejects the ~300 MB motivating payload, file/config.py:75)
6. Use the dead File header FLAGS field (backend.py:235, hardcoded 0) to record "uncompressed Arrow, mmappable"

Constraints

• Plaintext only. Encrypted values (@cache.secure) can never be mmap-read — AES-GCM decrypt reads the whole ciphertext into an owned buffer. Keep zstd for encrypted.
• Depends on SerializationWrapper.unwrap copies the full payload on every read (incl. every L1 hit) #162. Sequence after the correctness tranche (Legacy RedisBackend corrupts all binary payloads (decode_responses=True) #154–No-pyarrow columnar fallback crashes on pandas nullable extension dtypes (Int64/Float64) #160).