feat(format): apply fill values to unallocated storage on read

HDF5 allocates lazily: a chunk nobody wrote doesn't exist in the file, and a
dataset nobody wrote has no data address. Such regions must read as the
dataset's fill value. There was no Fill Value message parser at all, so:

- a sparse chunked dataset read its holes as zeros — silently wrong whenever
  the fill value isn't zero (h5py `fillvalue=-1` came back as 0);
- a dataset that was created but never written failed with NoDataAllocated /
  "no address for chunked layout" where h5py returns a filled array.

New clawhdf5_format::fill_value: parses Fill Value messages v1-v3 and the old
0x0004 message (validated against HDF5 2.0 output under default and latest
libver), builds a fully filled dataset when there is no storage, and writes the
fill value into exactly the chunk-grid cells absent from the chunk index —
never mistaking a stored zero for a hole, clipping edge chunks, any rank. It is
skipped entirely for the default (zero) fill value. The chunk index dispatch is
extracted from read_chunked_data into a reusable list_chunks.

The reader, lazy and mmap facades apply it on full reads; selection reads go
through a fill-aware full read when the fill value matters. h5py interop test
compares against h5py's own readback, including a sparse 2-D dataset and a
hyperslab straddling allocated and unallocated chunks.

Co-Authored-By: Claude Fable 5.1 <[email protected]>

crates/clawhdf5-format/src/lib.rs

@@ -67,6 +67,7 @@ pub mod ea_writer;
 pub mod error;
 pub mod extensible_array;
 pub mod file_writer;
+pub mod fill_value;
 pub mod filter_pipeline;
 pub mod filters;
 mod filters_szip;