BUG: preserve correlogram unit axes across segments

[AtomicGlance]

Fixes #4736.

When a unit is silent in one segment, the NumPy path sizes its segment result from the number of active units. With two units this can broadcast counts into the wrong pairs; non-contiguous indices can also raise an error. Pass the complete sorting's unit count to both segment helpers so each result preserves the global unit axes, including empty segments.

The optional num_units argument also affects direct calls to the exported helpers. Without it, the size is inferred as max(unit_index) + 1 (zero for empty input), leaving zero rows for missing indices. For example, labels [0, 2] now produce three auto-correlogram rows rather than two. This preserves index coordinates instead of dropping counts or raising for non-contiguous indices. I am open to using an accumulator argument instead if that is the preferred API.

The regression checks exact auto- and cross-correlogram counts for two and three units across segments, including an empty segment. It runs without Numba and additionally compares both implementations when Numba is installed. Both cases fail against the corresponding functions from upstream f03253e9c and pass with this change.

Validation on Python 3.13:

• Full test_correlograms.py module: 42 passed, 40 skipped.
• Black check on both changed files: passed.
• git diff --check: passed.

The test environment uses the development ProbeInterface dependency required by this checkout and its declared Zarr 2 / numcodecs constraints. Pytest plugin autoload was disabled to avoid loading an unrelated DANDI plugin from the host environment.

[alejoe91]

Since correlogram_for_one_segment and auto_correlogram_for_one_segment are only used internally and not exposed at the module level, let's just make num_units mandatory to avoid any confusion

[AtomicGlance]

Done in 478f0dc. Both internal helpers now require num_units, and the unique-index fallback is removed. I also updated the docstrings to say int and adjusted the existing helper test to pass the complete unit count explicitly.

The correlogram test module passed locally: 43 passed, 40 skipped. Black and git diff --check passed too. Pytest plugin autoload was disabled to avoid an unrelated DANDI plugin from the shared environment. CI is rerunning for this commit.