refactor: move alignment code to linopy/alignment.py; restructure tests (#742)

* refactor: move conversion/broadcasting/alignment code to linopy/alignment.py

Pure move, no behavior change. The new module owns the seam between user
input and linopy's labelled arrays:

- coords parsing: _coords_to_dict, _as_index, _as_multiindex
- conversion: get_from_iterable, pandas_to_dataarray, numpy_to_dataarray,
  _named_pandas_to_dataarray, fill_missing_coords, as_dataarray
- MultiIndex projection: _LevelProjection, _project_onto_multiindex_levels,
  _warn_implicit_projections
- broadcasting: _broadcast_to_coords, broadcast_to_coords
- validation: validate_alignment
- symmetric alignment: align

common.py keeps the general utilities (formatting, label indexes, polars
helpers, decorators). Importers (model, expressions, variables, __init__,
tests) updated; no re-exports.

Follow-up requested by @FabianHofmann in #732.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

* test: restructure alignment tests into test_alignment.py

One class per concept in linopy.alignment, mirroring the module's public
surface:

- TestAsDataarrayFrom{Pandas,Numpy,Scalar,DataArray} + MultiIndexCoords
- TestCoordsToDict (the coords-entry naming rules)
- TestBroadcastToCoords (strict=False mechanics)
- TestMultiIndexProjection (projection values, deprecation warnings,
  coverage gaps — the legacy/v1 fork point for #717)
- TestStrictMode (strict=True contract)
- TestValidateAlignment
- TestAlign

Shared fixtures (mi_index / mi_coords / by_level1) replace the repeated
MultiIndex setup; the pandas dims-naming and numpy labeling tests are
consolidated into parametrized tables. test_common.py keeps the utility
tests. Full suite count unchanged (3202) — no coverage lost.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

* test: close coverage gaps in alignment.py and the MI coords serialization

alignment.py: 97% -> 99%. New edge-case tests: bare-string dims, 0-d
arrays, fill_missing_coords type check, partially-named MI levels,
gap detection with extra dims, gap-error truncation (>5 missing
combinations). The two remaining uncovered lines are defensive branches
for inputs outside the DimsLike contract (non-iterable dims).

common.py: 88% -> 90%. The MultiIndex round-trip through
coords_to_dataset_vars / coords_from_dataset (used by CSRConstraint)
had zero coverage; now pinned.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

* Update test/test_alignment.py

Co-authored-by: Fabian Hofmann <fab.hof@gmx.de>

* Update test/test_alignment.py

Co-authored-by: Fabian Hofmann <fab.hof@gmx.de>

---------

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Co-authored-by: Fabian Hofmann <fab.hof@gmx.de>

Author: 3 people

doc/release_notes.rst

@@ -74,7 +74,7 @@ Most users should keep calling ``model.solve(...)``. If you want more control, y
 
 **Internal**
 
-* ``linopy.common`` provides two DataArray conversion helpers: ``as_dataarray`` (convert only) and ``broadcast_to_coords`` (convert and broadcast against ``coords``). The latter takes ``strict`` (default ``True``): any mismatch with ``coords`` raises, naming ``label`` in the error; ``strict=False`` passes mismatches through for downstream xarray alignment.
+* New module ``linopy.alignment`` owns conversion, broadcasting, and alignment of user input against coordinates (moved out of ``linopy.common``): ``as_dataarray`` (convert only), ``broadcast_to_coords`` (convert and broadcast against ``coords``; ``strict=True`` by default raises on any mismatch, naming ``label`` in the error), ``validate_alignment``, and ``align``.
 * Each ``Solver`` subclass now overrides at most three hooks: ``_build_direct`` (build the native model), ``_run_direct`` (run it), and ``_run_file`` (run the solver on an LP/MPS file). File-only solvers (CBC, GLPK, CPLEX, SCIP, Knitro, COPT, MindOpt) only override ``_run_file``.
 * New ``ConstraintLabelIndex`` cached on ``Model.constraints`` (mirrors the existing ``Variables.label_index``); ``ConstraintBase`` gains ``active_labels()`` and a ``range`` property; ``CSRConstraint`` exposes ``coords``.
 * ``linopy.common`` gains ``values_to_lookup_array``; the legacy pandas-based helpers ``series_to_lookup_array`` and ``lookup_vals`` are removed.

linopy/__init__.py

@@ -12,7 +12,7 @@
 # Note: For intercepting multiplications between xarray dataarrays, Variables and Expressions
 # we need to extend their __mul__ functions with a quick special case
 import linopy.monkey_patch_xarray  # noqa: F401
-from linopy.common import align
+from linopy.alignment import align
 from linopy.config import options
 from linopy.constants import (
     EQUAL,