refactor: Switch Arrow/Orjson serializers from Blake3 to xxHash3

[27Bslash6]

Summary

Arrow and Orjson serializers use checksums for integrity checking. This issue tracks alignment with the Rust ByteStorage layer.

Current State (2025-12-11)

✅ Phase 1 Complete: Switched to xxHash3-64 via Python xxhash package

Serializer	Checksum	Size	Implementation
StandardSerializer	xxHash3-64	8 bytes	Rust ByteStorage (FFI)
ArrowSerializer	xxHash3-64	8 bytes	Python xxhash package
OrjsonSerializer	xxHash3-64	8 bytes	Python xxhash package

Files updated:

• src/cachekit/serializers/arrow_serializer.py
• src/cachekit/serializers/orjson_serializer.py
• Tests: test_xxhash_integrity.py (14 new tests), updated existing tests

Future Work: FFI Implementation

🔮 Phase 2 (Optional): Use Rust FFI for checksums instead of Python package

Blocked by: cachekit-io/cachekit-core#13 (checksum-only API)

# Current (Python xxhash)
import xxhash
checksum = xxhash.xxh3_64_digest(data)

# Future (Rust FFI) - requires cachekit-core#13
from cachekit._rust_serializer import compute_checksum
checksum = compute_checksum(data)

Benefits of FFI approach:

• Single implementation (no Python xxhash dependency)
• Consistent with StandardSerializer path
• Potentially faster for large payloads (avoid Python GIL)

Trade-offs:

• FFI overhead may negate speed gains for small payloads
• More complex build (Rust required)
• Current Python solution works fine

Decision Log

• 2025-12-11: Implemented Phase 1 (Python xxhash). Phase 2 deferred pending cachekit-core#13 and benchmarking to determine if FFI overhead is worth it.

Related

• Upstream: cachekit-core#13 (checksum-only API in Rust)
• Context: xxHash3 migration in ByteStorage (2025-12-05)