Integrate longitudinal template generation into rbc (Stage 2 of #301)

Ports scripts/build_robust_template.py into the rbc package layout so the
template stage matches the rest of the pipeline and gets test coverage:

- core/longitudinal/freesurfer.py: generate_robust_template +
  fs_to_itk_xfm via niwrap freesurfer + rbc.core.fsl2itk (no c3d).
- workflows/longitudinal/template.py: LongitudinalTemplateOutputs +
  generate_subject_template().
- bids/longitudinal/template.py: discover_template_inputs +
  export_template; xfm output naming switched to
  sub-X_ses-longitudinal_from-Y_to-longitudinal_mode-image_xfm.txt.
  resolve_longitudinal_{anat,func} updated to query by from+to extras.
- orchestration/longitudinal/template.py: per-subject loop. Fixes #19
  (per-subject volume check now inside the loop, not on the whole df).
- cli/longitudinal/ becomes a package with nested subparsers and a
  ``long`` alias. ``rbc longitudinal template`` is the new subcommand;
  the legacy --anatomical/--functional flow lives at
  ``rbc longitudinal process`` until Stage 3 splits it.
- noit=True ported as-is with TODO referencing #302.
- Drops the FS license requirement entirely: orchestration sets the
  documented SURFER_SIDEDOOR bypass on the runner environ, since
  mri_robust_template's chklc() returns immediately when the var is set.
- Deletes scripts/build_robust_template.py.

Unit tests cover filename mapping, the per-subject single-volume guard,
template input discovery, BIDS export naming, and the new CLI wiring
(including the ``long`` alias).

Tier-2 integration test for ``rbc longitudinal template`` deferred:
needs the ds000114 multi-session fixture infrastructure (download
script + CI cache + conftest), which is substantial enough to land
in a separate PR. Existing ``pytest -m 'integration and not slow'``
collects 0 tests today, so no regression risk.

Author: nx10

scripts/build_robust_template.py

@@ -4,7 +4,7 @@
 
 from typing import TYPE_CHECKING
 
-from rbc.bids import Suffix, TemplateSpace
+from rbc.bids import Suffix, TemplateSpace, bids_safe_label
 
 if TYPE_CHECKING:
     from pathlib import Path
@@ -48,7 +48,7 @@ def resolve_longitudinal_anat(
             tpl_df,
             suffix="xfm",
             extension=".txt",
-            extra={"from": ses},
+            extra={"from": bids_safe_label(ses), "to": "longitudinal"},
         ),
         "brain": anat_q.expect(anat_df, suffix=Suffix.T1W, desc="brain"),
         "brain_mask": anat_q.find(anat_df, suffix=Suffix.MASK, desc="T1w"),

src/rbc/bids/longitudinal/anatomical.py

@@ -4,7 +4,7 @@
 
 from typing import TYPE_CHECKING
 
-from rbc.bids import Suffix
+from rbc.bids import Suffix, bids_safe_label
 
 if TYPE_CHECKING:
     from pathlib import Path
@@ -41,7 +41,7 @@ def resolve_longitudinal_func(
             tpl_df,
             suffix="xfm",
             extension=".txt",
-            extra={"from": ses},
+            extra={"from": bids_safe_label(ses), "to": "longitudinal"},
         ),
         "bold_to_anat_itk": func_q.expect(
             func_df,