refactor: move the microsim.psf implementation to private microsim._psf

[tlambert03]

Pure move, no behavior change: the implementation of microsim.psf now lives in a private microsim._psf, and microsim.psf re-exports the same public names as before (make_psf, make_confocal_psf, cached_psf, vectorial_*...). The first commit is just the git mv, if you want to check that nothing else changed.

This is prep for an upcoming PR that reworks the confocal PSF pipeline (excitation spot computed at the excitation wavelength), which adds a bunch of internal helpers that I don't want to become public API. I'd like to move more of the accidentally-public stuff out of microsim.psf eventually.

[codecov]

Codecov Report

❌ Patch coverage is 95.50562% with 8 lines in your changes missing coverage. Please review.
✅ Project coverage is 88.26%. Comparing base (da349b2) to head (a747bf3).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/microsim/_psf.py	95.42%	8 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##             main     #162       +/-   ##
===========================================
+ Coverage   67.26%   88.26%   +21.00%     
===========================================
  Files          48       49        +1     
  Lines        3314     3316        +2     
===========================================
+ Hits         2229     2927      +698     
+ Misses       1085      389      -696     

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