improve hash function in client_hash

[jaromil]

The hash function uses a simple bitmask of the file descriptor value (fd & 0x3FF), which is a weak hash for file descriptors. File descriptors are typically allocated sequentially starting from 3, meaning the first 1024 clients will have nearly perfect distribution, but if fds wrap around or are reused with certain patterns, collisions will increase.

While this is likely acceptable for the expected workload, consider using a better hash function (e.g., multiply by a prime and shift) for more uniform distribution. For example: ((fd * 2654435761u) >> 22) provides better mixing of bits.

    /* Use multiplicative hashing for better distribution of sequential fds */
    uint32_t ufd = (uint32_t)fd;
    uint32_t h = (ufd * 2654435761u) >> (32 - CLIENT_HASH_BITS);
    return h & CLIENT_HASH_MASK;

Originally posted by @Copilot in #29