…1715)

The api-consistency sweep on 2026-05-12 found ``write_vrt`` was the only
writer in ``xrspatial.geotiff`` using ``crs_wkt`` instead of ``crs``:

    to_geotiff(data, path, *, crs: int | str | None = None, ...)
    write_geotiff_gpu(data, path, *, crs: int | str | None = None, ...)
    write_vrt(vrt_path, source_files, *, crs_wkt: str | None = None, ...)

This broke the "forward the same kwargs to whichever writer matches the
output extension" pattern. Generic write-wrapper code had to special-case
VRT, convert EPSG int -> WKT for that path, and remember that one writer
in the trio used a different name. Reader-side ``attrs['crs']`` is the
canonical EPSG-int surface, so the asymmetry is also visible in
round-trip code.

This change adds ``crs`` as the canonical kwarg with the same
``int | str | None`` shape as the sibling writers. ``crs_wkt`` is kept
as a deprecated alias for backward compatibility:

* ``crs=`` accepts int (EPSG), str (WKT or PROJ), or None.
* ``crs_wkt=`` still works but emits ``DeprecationWarning``.
* Passing both raises ``TypeError`` rather than silently picking one.
* Omitting both (the most common call shape) emits NO warning.

The wrapper normalises any non-None input through the new
``_resolve_crs_to_wkt`` helper before forwarding to the internal
``_vrt.write_vrt`` (which only speaks WKT). EPSG int and PROJ-string
inputs are routed through pyproj's ``CRS.from_user_input(...).to_wkt()``;
already-WKT strings (recognised by the PROJCS/GEOGCS/PROJCRS/GEOGCRS/
COMPD_CS/COMPOUNDCRS prefix) pass through verbatim so the existing
WKT-passthrough behaviour for the XML-escape tests is preserved.

The deprecation shim follows the same sentinel-default pattern the
deprecated ``gpu=`` alias uses on ``read_geotiff_gpu``: a private
``_CRS_WKT_DEPRECATED_SENTINEL`` distinguishes "user passed nothing"
(silent) from "user passed ``crs_wkt=None`` explicitly" (warn).

test_write_vrt_crs_1715.py adds 12 tests pinning the new contract:

* signature pins (``crs`` present, default None, annotation
  ``int | str | None``);
* runtime: ``crs=<EPSG int>``, ``crs=<WKT string>``, ``crs=None``,
  no-crs-kwarg-no-warning, all round-trip back to ``attrs['crs']``;
* deprecation shim: ``crs_wkt=<wkt>`` warns + still works,
  ``crs_wkt=None`` (explicit) warns, both-kwargs raises ``TypeError``;
* cross-writer parity: ``crs`` annotation matches on all three writers;
* negative tests: invalid type / unparseable string raise on the public
  wrapper.

Existing tests that were calling ``write_vrt(..., crs_wkt=...)`` through
the public wrapper are migrated to ``crs=`` so the suite no longer emits
spurious DeprecationWarnings:

* ``test_polish_1488.py``: ``crs=None`` replaces ``crs_wkt=None``.
* ``test_kwarg_behaviour_2026_05_12.py::TestWriteVrtCrsWktBehaviour``:
  three sub-tests migrated from ``crs_wkt=`` to ``crs=``.
* ``test_signature_parity_1631.py``: signature test updated to assert
  both kwargs and uses ``crs=None`` in the runtime call.

``test_vrt_xml_escape_1607.py`` keeps using ``crs_wkt=`` because it
imports from the internal ``xrspatial.geotiff._vrt`` (not the public
wrapper) and exercises raw XML-escape semantics; the rename does not
apply there.

Closes #1715.

- Drop unused ``open_geotiff`` import from test_write_vrt_crs_1715
- Fix stale ``test_namespace_no_leak_1715.py`` reference in the
  test_signature_parity_1631 docstring; the deprecated ``crs_wkt``
  alias is exercised in test_write_vrt_crs_1715.py
- Correct write_vrt ``crs_wkt`` docstring: the alias is ``str | None``
  and runs through ``_resolve_crs_to_wkt`` (so WKT, PROJ strings, and
  ``EPSG:NNNN`` all work), not WKT-only