Mitigate hashtable bucket collision attacks

[3em0]

Summary

• Add a lightweight 64-bit key mixer for hashtable bucket selection.
• Seed each generated table instance from its table/array addresses so raw attacker-chosen numeric patterns such as offset + i * bucket_count no longer map directly to the same bucket.
• Preserve the table seed across rehashing.
• Add a table test covering keys that would collide under raw modulo bucket assignment.

Fixes #119.

Testing

• git diff --check
• Verified the deterministic test key set with the same 64-bit mixing arithmetic.

Full C unit tests were not run because this environment does not have a C compiler installed (gcc, cc, clang, and tcc are unavailable).