clawhdf5-netcdf4: variables' dimensions come from the file

Variables got the first unused dimension of equal size, so a variable on
an unlimited dimension with fewer records got an anonymous dim_<n>, and
dimensions of one size could be swapped. Resolve them as netCDF-C does
(libhdf5/hdf5open.c): _Netcdf4Coordinates ids, else the scales
DIMENSION_LIST references (the last one attached to an axis), searched in
the variable's group and its parents; a coordinate variable is on its own
scale. Size matching remains only for axes the file names nothing for.

variables()/variable_names() leave out dimension scales that are only
dimensions, and _nc4_non_coord_<name> is the variable <name>.
Variable::shape is the netCDF shape (an unlimited dimension's length) and
the reads pad unwritten records with the fill value (_FillValue, else
NC_FILL_*; NaN from read_f64); Variable::stored_shape is the HDF5 extent.
New NetCDF4File::variable_names.

Tests compare with netCDF4-python variable by variable: the known-issues
reproducer, equal sizes, (p, p), scalars, inherited dimensions, unwritten
records, h5py dimension scales, h5netcdf and xarray files. CI installs
h5netcdf. known-issues entry moved to Fixed (history); stale open-table
row for the unlimited-size fix removed.

Co-Authored-By: Claude Opus 5.5 (1M context) <[email protected]>

README.md

@@ -411,7 +411,8 @@ scripts/ci-test.sh                   # what CI runs: fmt, clippy matrix, tests,
 conformance/run.sh                   # the conformance report (needs h5py, hdf5plugin, h5dump)
 ```
 
-The interop suites need a Python with h5py (and netCDF4, xarray); on a
+The interop suites need a Python with h5py (and netCDF4, xarray; the
+NetCDF-4 tests of h5netcdf-written files skip without h5netcdf); on a
 PEP 668 system that has to be a virtualenv, which `ci-test.sh` finds as
 `.venv` or through `CLAWHDF5_PYTHON`. Without one they skip; set
 `CLAWHDF5_REQUIRE_INTEROP=1` to make that a failure, as CI does: