Make Setas a descriptor; add ColumnHeadersDescriptor; merge stable

[Copilot]

The setas attribute was implemented as a @property in two separate places (DataArray, DataFilePropertyMixin), scattering the get/set logic outside the Setas class. column_headers had the same problem. This PR makes Setas itself a Python descriptor and introduces ColumnHeadersDescriptor, then merges the stable branch (which independently made DataArray a descriptor for the data attribute).

Setas as a descriptor (setas.py)

• Added __set_name__, __get__, __set__ to Setas.
• __get__ retrieves the per-instance Setas stored as _setas, shape-syncing on access.
• __set__ delegates to Setas.__call__, preserving all existing assignment call-convention logic.
• New optional source kwarg: Setas(source="_data") makes get/set forward through getattr(obj, source).setas — used by DataFilePropertyMixin to reach its inner DataArray.

ColumnHeadersDescriptor (setas.py)

New slim descriptor exposing column_headers uniformly via obj.setas.column_headers.

DataArray (array.py)

Replaced the setas and column_headers @property pairs with class-level descriptors:

class DataArray(ma.MaskedArray):
    setas = _setas()                        # Setas descriptor; private store: _setas
    column_headers = ColumnHeadersDescriptor()

Also incorporates DataArray.__set_name__/__get__/__set__ from stable, enabling DataArray itself to act as a descriptor for the data attribute on DataFilePropertyMixin.

DataFilePropertyMixin (property.py)

Replaced the setas, column_headers, and data @property definitions with class-level descriptors:

class DataFilePropertyMixin:
    setas = Setas(source="_data")       # delegates to self._data.setas
    column_headers = ColumnHeadersDescriptor()
    data = DataArray([])                # DataArray descriptor; private store: _data

The data descriptor's __set__ handles the shape normalisation and setas/column-header preservation previously done by the @property data setter.

Merge with stable

One conflict in property.py resolved by keeping our setas/column_headers descriptors and stable's data = DataArray([]) form, dropping both the old @property data getter/setter and the intermediate @property column_headers.

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Test Results

    4 files  ±0      4 suites  ±0   25m 51s ⏱️ +9s
  332 tests ±0    332 ✅ ±0  0 💤 ±0  0 ❌ ±0 
1 328 runs  ±0  1 328 ✅ ±0  0 💤 ±0  0 ❌ ±0 

Results for commit c9ed35e. ± Comparison against base commit fd2aae8.