performance: Replace DJB2 with xxHash (#3)

[AlexJuca]

Context

The current hash function is DJB2. Replacing it with xxHash or another high-quality fast hash may improve distribution and throughput, especially as table sizes and workloads grow.

Scope

• Benchmark DJB2 against xxHash or another candidate using representative FKVS key distributions.
• Evaluate collision behavior, CPU cost, and dependency impact.
• Decide whether to vendor a small implementation or add a dependency.
• Keep hash behavior deterministic across supported platforms.

Acceptance criteria

• Benchmark data justifies the replacement.
• Collision behavior improves or remains acceptable across realistic key sets.
• Tests cover basic hash-table correctness after replacement.
• The change does not regress portability or build simplicity.