Step 10: XXH64 #4

Design §8 step 10, from docs/design.md as the owner ruled it on 2026-09-25. The step's entry in design §8 records what each check printed when it passes; this issue tracks the work.

Step 10: XXH64. From xxHash's specification document, copied into docs/specs/ with its SHA-256 (decision 18).

Check: the ruled specification's test values; equal to libzstd's checksums through the oracle; throughput; mutations.

Waits on: step 2, for libzstd as the oracle.