FunctionalLongOutputs gains regressed_bold and cleaned_bold dicts
keyed by regressor strategy.  longitudinal_process now accepts
regressor_files (raw .1D from cross-sectional) and re-runs
apply_regression + apply_regression_bandpass on the warped BOLD.
No regressor recomputation -- same matrix, different target space.

bold_mask is now mandatory (was Path | None).

BIDS export writes per-regressor desc-regressed and desc-preproc BOLD
with reg-<strategy> entity.  resolve_longitudinal_func resolves the
raw regressor files per strategy.

CLI gains --regressor with the same choices as cross-sectional
(36-parameter, aCompCor).

Port of the longitudinal regression logic from #282.

Co-authored-by: Janhavi Pillai <janhavi.pillai@gmail.com>

Unit tests: resolve_longitudinal_func with single/multiple regressors,
missing regressor raises, mandatory bold_mask; export file counts and
reg-<strategy> entity validation.

Tier-2 integration: apply_regression and apply_regression_bandpass on
warped BOLD with raw regressors produce non-degenerate outputs.

Tier-4 full_pipeline: longitudinal functional outputs exist, regressed
and cleaned BOLD have non-zero variance, bold mask is binary.

… integration

- Drop regressor_set param from longitudinal_process; iterate
  regressor_files.keys() instead.  The resolve layer already filters to
  the requested strategies, so the dict keys ARE the set.  Eliminates
  the caller sync burden.

- Delete tests/full_pipeline/longitudinal/ -- the conftest hacked the
  cross-sectional anat brain as a "longitudinal template", which
  exercises the code path but not the actual transform chain.

- Rewrite tests/integration/longitudinal/test_regression_reuse.py as a
  proper end-to-end CLI test: rbc functional -> rbc longitudinal
  functional on ds000114 sub-01 ses-test via subprocess.  Asserts
  expected BIDS tree, non-degenerate variance, and binary mask.

- Add ds000114_func_derivatives and longitudinal_func_output fixtures
  to the integration conftest so the full cross-sectional -> longitudinal
  chain runs on real multi-session data with docker.

Filters.apply() applies the --task filter to ALL rows including anat.
Anat files have task=null, so --task fingerfootlips drops them,
causing resolve_functional to fail with FileNotFoundError on
desc-brain_T1w.

Drop --task from both rbc functional and rbc longitudinal functional
fixture calls.  ds000114 only has one task per session so the filter
isn't needed.  Remove debug logging from previous commits.

The --task filter applied pl.col("task") == value to all rows, but
anat/dwi files have task=null, so they were silently dropped.  This
caused rbc functional --task <label> to fail with FileNotFoundError
on desc-brain_T1w for any dataset.

Fix: use pl.col("task").is_null() | (pl.col("task") == value) so
rows without a task entity (anat, dwi, fmap) pass through.

Affects functional and all orchestration; metrics/qc pre-filter to
datatype=func so they were never hit.

Restore --task in integration fixtures now that the fix is in place.