ENH: Upstream BIDSSourceFile and CreateFreeSurferID interfaces from fMRIPrep

[oesteban]

Summary

fMRIPrep contains anatomical pipeline interfaces in `fmriprep/interfaces/bids.py` that are fundamentally sMRIPrep concerns:

`BIDSSourceFile` (lines 80-96) + `_create_multi_source_file` (lines 120-161)

Determines the BIDS source file for anatomical derivatives, handling multi-image and sessionwise cases. This is an anatomical pipeline concern — determining which T1w/T2w is the source file for multi-image pipelines. sMRIPrep's `init_anat_fit_wf` needs equivalent logic.

`CreateFreeSurferID` (lines 108-117) + `_create_fs_id` (lines 164-190)

Creates FreeSurfer subject IDs from BIDS subject/session identifiers (e.g., `"01"` -> `"sub-01"`, with optional session suffix). This interface already imports `stringify_sessions` from sMRIPrep, confirming it belongs here.

Code to upstream

• `BIDSSourceFile` interface + `_create_multi_source_file` helper
• `CreateFreeSurferID` interface + `_create_fs_id` helper

Rationale

• Both interfaces handle anatomical/FreeSurfer pipeline concerns, not fMRI-specific logic
• `CreateFreeSurferID` already imports from sMRIPrep (`stringify_sessions`)
• These belong alongside sMRIPrep's anatomical fit workflow
• dMRIPrep and NiBabies need the same source file resolution and FreeSurfer ID management

Priority

MEDIUM.