use FNV-1a as documented in rendezvous hashing

[geyingqi (mowangdk)]

Summary

The comment on Hasher states the implementation is based on FNV-1a, but fnvhash used fnv.New64() which is FNV-1. Switch to fnv.New64a() to match the documented algorithm and get better distribution.

Why this is safe

The hash function is used solely for live work assignment across controller replicas via rendezvous hashing. No hash values are persisted anywhere.

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[gemini-code-assist]

Code Review

This pull request changes the hashing function in rendezvous.go to use fnv.New64a() instead of fnv.New64(). The reviewer points out that using fnv.New64a() with a string-to-byte-slice conversion causes heap allocations, and suggests implementing the FNV-1a algorithm inline to improve performance.