64-bit hash function for UltraLogLog.

v0.1 ships with :erlang.phash2/2 upgraded to 64 bits via concatenation. This is fine for correctness testing but not statistically ideal for production sketches at high precision — phash2 is not a high-quality hash for cardinality estimation.

v0.2 plan

• Pure-Elixir xxhash3 (slow but no NIF) as fallback.
• Optional Rustler NIF for xxhash3 / komihash5 (matches Hash4j reference).
• Pluggable behaviour so callers can pass pre-computed hashes from whatever they already use.

Why 64 bits

UltraLogLog needs 64 bits because:

• top p bits select the register (up to p=26)
• remaining bits contribute to the geometric leading-zero variable
• at exa-scale (~10^18 distinct items) 64 bits are exactly what's needed to keep collision probability negligible (Heule++ argument)