ENH: add spline interpolation motion correction for fNIRS (motion_correct_spline)

[leonardozaggia]

Reference issue (if any)

Nonne (new feature)

What does this implement/fix?

Adds motion_correct_spline (alias spline) to mne.preprocessing.nirs for spline interpolation-based motion correction of fNIRS data.

For each detected motion-artifact segment the signal is detrended with a smoothing spline, then consecutive segments are baseline-shifted so they connect smoothly. Accepts a per-channel artifact mask tIncCh; when None, only spline detrending is applied with no baseline shifting.

Based on Homer3 v1.80.2 hmrR_tInc_baselineshift_Ch_Nirs (Huppert et al. 2009).

Additional information

• Operates on optical density or hemoglobin data.
• p controls spline smoothness (smaller = tighter fit to data).
• tIncCh is shape (n_picks, n_times); True = clean, False = artifact.
• This is one of three related PRs adding Homer3-based motion correction to mne.preprocessing.nirs
  - PCA: ENH: add PCA-based motion correction for fNIRS (motion_correct_pca) #13691
  - Spline: this PR
  - Wavelet: ENH: add wavelet-based motion correction for fNIRS (motion_correct_wavelet) #13692

[larsoner]

@leonardozaggia I wonder if these should live in https://github.com/mne-tools/mne-nirs/tree/main/mne_nirs/preprocessing instead... we can iterate and release faster over in mne-nirs than we can here, and at some point I think decided we should try to put newer functionality there when possible. WDYT?

[leonardozaggia]

@arsoner thanks for your feedback.
I considered this point when first working on this PR. At the end I opted for mne-python because the only other motion regression technique available - TDDR - lives here and thought it best for users to have the motion correction techniques in the same place.

[leonardozaggia]

But of course, I am open to cancel my PR and submit a new one to the dedicated mne-nirs repo if you think it's best :)