clawhdf5-netcdf4: phony dimensions, skipped types and order as netCDF-C

Read a file's metadata the way netCDF-C 4.9.3 does (libhdf5/hdf5open.c),
for the whole file on first use (src/model.rs, replacing src/scope.rs):

- links in creation order when the group tracks it, else name order;
  a group's datasets before its subgroups; dimension ids file-wide;
- variables' dimensions from _Netcdf4Coordinates (file-wide ids), else
  the scales DIMENSION_LIST attaches when the first axis has one, else
  netCDF-C's phony dimensions phony_dim_<id> (create_phony_dims: shared
  by length and unlimitedness within a group, not between two axes of
  one variable, numbered subgroups first, a zero length unlimited);
- datasets of types netCDF-C cannot represent are not variables
  (references, bit fields, time, arrays, compounds/enums/VLENs over
  them), replaying netCDF-C's file-wide type list, failed types
  included;
- unlimited lengths as nc4_find_dim_len (its group and below).

NcType gains Enum, Compound, VLen, Opaque and is #[non_exhaustive];
Variable::nc_type is netCDF-C's type (1-byte strings NC_CHAR). New
clawhdf5_format::group_v2::links_in_creation_order_in.

Tests compare with netCDF-C itself (tests/netcdf_c_view.py calls the
libnetcdf netCDF4-python bundles through ctypes): new interop cases for
h5py files without dimension scales, every type class, link order; and
the gated corpus_vs_netcdf_c (CLAWHDF5_NETCDF_CORPUS): 420 of the 429
conformance-corpus files netCDF-C opens match (main: 68); the other 9
are explained in tests/corpus_known_differences.txt and known-issues.

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

README.md

@@ -386,7 +386,7 @@ filter).
 | `clawhdf5-filters` | Deflate backends (zlib-rs default, zlib-ng, Apple Compression) |
 | `clawhdf5-io` | I/O helpers: mmap, async, an HSDS client, `mpi-io` (not collective I/O) |
 | `clawhdf5-remote` | HTTP(S) and object-store files through a block cache |
-| `clawhdf5-netcdf4` | NetCDF-4 dimensions, variables, CF attributes |
+| `clawhdf5-netcdf4` | NetCDF-4 dimensions, variables, CF attributes, as netCDF-C reports them (phony dimensions included) |
 | `clawhdf5-derive` | Derive macros for HDF5-serialisable structs |
 | `clawhdf5-tools` | `h5rs`: `ls`, `dump`, `stat`, `diff`, `check` |
 | **Bindings** | |