BUG: Typo in swarmsplitter.check_CCG that disables the refractoriness gate

[HazenBabcock]

Describe the issue:

I think that:

if len(st1) == 0 or len(st2 == 0) or T == 0:
        return False, False

Should be:

if len(st1) == 0 or len(st2) == 0 or T == 0:
        return False, False

Reproduce the bug:

This issue was identified by Claude Code which I was using to better understand how Kilosort4 merges/splits units.

Error message:

Version information:

Python 3.12.3, Kilosort HEAD.