Fix datetime day filtering

[driavysinus]

Problem

filter_datetime(..., day=...) currently reads the year component when applying the day filter, so day-of-month slicing can incorrectly return no matches.

Fix

Use pc.day(...) for the day branch and add a focused unit test for day-of-month filtering.

Validation

Z:\uv\uv.exe run --with datasets==2.21.0 --with pytest --with pandas --with pyarrow --with numpy --with psutil python -m pytest tests\cyclops\data\test_slicer_datetime.py -q
python -m py_compile cyclops\data\slicer.py tests\cyclops\data\test_slicer_datetime.py

Note: I used datasets==2.21.0 locally to match the repository's Poetry constraint range and avoid newer datasets API drift.