reproject: rename vertical CRS kwargs to source_/target_vertical_crs (#2613)

[brendancol]

Closes #2613

• Renames src_vertical_crs / tgt_vertical_crs to source_vertical_crs / target_vertical_crs so the vertical-datum kwargs match the source_crs / target_crs spelling already used in the same signature.
• Keeps the old names working through a deprecation shim: passing one emits a DeprecationWarning, and passing both the old and new spelling for the same side raises TypeError.
• Updates the docstring and migrates the existing vertical-shift tests to the new names.

Backend coverage: the signature is shared across backends (numpy, cupy, dask+numpy, dask+cupy); verified the new and deprecated names on numpy and cupy entry points.

Test plan:

• pytest xrspatial/tests/test_reproject.py::TestVerticalShift (14 passed)
• cupy smoke test: new names and deprecated-alias warning both produce identical output

[brendancol]

PR Review: reproject vertical CRS kwarg rename

Blockers (must fix before merge)

None.

Suggestions (should fix, not blocking)

None.

Nits (optional improvements)

• Passing the deprecated name with an explicit None (e.g. src_vertical_crs=None) still emits the DeprecationWarning, since the shim treats any non-sentinel value as "the old name was used" (_resolve_deprecated_vertical_kwarg in reproject/__init__.py). That is defensible: the caller did type the old name.

What looks good

• The sentinel-based shim tells "not passed" apart from an explicit None, so the old and new defaults stay independent.
• Passing both spellings for one side raises TypeError, and the new test covers it.
• stacklevel=3 points the warning at the user's call site rather than the internal helper.
• The new and old names were verified on both the numpy and cupy entry points and produce identical output.
• No other call sites in xrspatial, docs, or examples use the old names, so the rename is self-contained.

Checklist

• Algorithm unchanged (signature/shim only)
• Implemented backends consistent (numpy + cupy verified)
• NaN handling unchanged
• Edge cases covered: deprecation warning, both-names conflict
• No premature materialization
• Benchmark not needed (no compute change)
• README feature matrix not applicable
• Docstrings updated for new and deprecated params