`.ipc._shm._shorten_key_for_macos()` may cause collisions for short names?

[goodboy]

From copilot's review in #406.

I'm adding this as a follow up since what we have in #406 is more
then good enough imo for a first macos-support attempt and given that
the .ipc._shm subsys is currently undocumented and still considered
somewhat unrefined/experimental.

---

_shorten_key_for_macos() ignores the provided prefix/suffix
when len(key) <= max_len (it returns key unchanged). This causes
collisions on macOS in _make_token() because shm_name,
shm_first_index_name, and shm_last_index_name can become
identical for “short” keys, and it also drops the prefix for
open_shm_list(). Adjust the early-return logic to include `prefix

• key + suffixand ensure the length check accounts for the leading '/' added bySharedMemory` on POSIX as well as the prefix/suffix
  lengths.

    # Account for the leading '/' added by POSIX SharedMemory names
    usable_len: int = max_len - 1
    full_key: str = f"{prefix}{key}{suffix}"
    if len(full_key) <= usable_len:
        return full_key

    _hash: str = hashlib.sha256(
        key.encode()
    ).hexdigest()

    hash_len: int = (
        usable_len