OxfordASLSplitM0
================

**Purpose.** Split a 4D ASL series into **ASL-only** and **M0** when ``ASLContext`` contains ``"m0scan"`` frames.  
If no ``m0scan`` exists, pass through the original ASL and use a **separate** M0 from ``data_descrip``.

Inputs
------

- Either from ``params``:
  - ``in_asl`` (explicit ASL path), or
- From ``data_descrip["Images"][<sub>]["asl"][0]``.
- ASL context, provided by (priority):
  1. ``data_descrip["ASLContext"]``
  2. ``params["asl_context"]``
  3. JSON file at ``perf/aslcontext.json`` (keys ``ASLContext``/``AslContext``)

Outputs
-------

- Writes into **derivatives mirror** (by default):
  - ``perf/asl_only.nii.gz``
  - ``perf/m0.nii.gz``
- Returns: ``{"asl_path": <out_asl>, "m0_path": <out_m0>}``.

Configuration
-------------

===================  ===================  ==========================================
Key                  Type / Default       Meaning
===================  ===================  ==========================================
``in_asl``           str / *auto*         Input ASL path (abs or BIDS-like)
``asl_context``      list[str] / *None*    Context list if not in metadata
``asl_context_json`` str / ``perf/aslcontext.json``  Fallback JSON
``out_asl``          str / ``perf/asl_only.nii.gz``  Output ASL
``out_m0``           str / ``perf/m0.nii.gz``        Output M0
===================  ===================  ==========================================

YAML example
------------

.. code-block:: yaml

    modules:
      - name: OxfordASLSplitM0