Documentation accuracy sweep: mmap, Blake3, pickle, orjson, master-key env var

[27Bslash6]

Summary

Several user-facing docs/strings describe behavior that does not exist:

• "zero-copy memory-mapped reads (lowest read memory)" for compression=none — no mmap path exists anywhere (settings.py:124, arrow_serializer.py:88-90,142,222). See EPIC: Zero-copy mmap read path for the File backend (large-object read RSS) #171.
• "Blake3 checksums" — ByteStorage uses xxHash3-64 (rust/src/lib.rs:46,52 + CLAUDE.md/DEVELOPMENT.md). BLAKE3 is only the cache-key hash. (core doc issue tracks the Rust side.)
• "plain pickle" for auto no-integrity mode — it is plain msgpack, no pickle anywhere (serializers/__init__.py:75). Misleads on both security surface and object support.
• orjson docstring claims "compression handled by Rust layer if enabled" — orjson never touches ByteStorage (orjson_serializer.py:141).
• Master-key error/docstring says REDIS_CACHE_MASTER_KEY; the real var is CACHEKIT_MASTER_KEY (encryption_wrapper.py:131, cache_handler.py:292).

Fix

Correct each string to match implemented behavior (or, for mmap, gate the claim on #171 landing).