feat: implement filter codecs, format write extensions, and MPI-IO VOL

All three SDD plans fully wired and committed to main:

Filter Codecs (FC):
- FC-1: Implement float E-scale in scaleoffset_decompress (value = minval +
  code * 2^E, negative exponents via cast to i32); add two round-trip tests.
- FC-2: filters_szip.rs — feature-gated SZIP decode via libaec FFI; SZIP
  dispatch arm added to decompress_chunk.
- FC-3: libaec-sys workspace crate with pkg-config probe and aec_buffer_decode
  FFI binding; added to workspace members.

Format Write Extensions (FWE):
- FWE-1: GroupBuilder::add_external_link() API; wired through FinishedGroup
  → GrpFlat → file_writer pass 1/2/3 (OH size, layout cursor, final write);
  external_link_write_roundtrip test.
- FWE-2: data_layout_write.rs — serialize_vds_mappings with length_size param
  and version 0/1 (external vs same-file) selection; declared as pub mod.
- FWE-3: with_virtual_sources empty-mapping guard (Important #9) — empty vec
  is silently ignored; vds_empty_mapping_list test updated to assert non-VDS
  layout results.

MPI-IO VOL Backend (MPI):
- MPI-1/2/3: mpi_vol.rs — MpiVol implementing VirtualObjectLayer; root-read
  + broadcast collective read; gather + root-write collective write; feature-
  gated mpi-io feature; wired into clawhdf5-io lib.rs.
- MPI-4: mpi_io_bench binary (h5bench-equivalent MPI-IO throughput bench).

mpi_vol.rs reviewer fixes:
- Doc-comment updated to accurately describe root-read+broadcast pattern
  (not MPI_File_read_at); MpiVol::expected_capabilities() associated fn
  added so tests can verify capabilities without a live MPI universe;
  rank_and_size_stub_values renamed to no_feature_error_contains_feature_name.

Workspace check: zero warnings, 20 test suites pass.

Co-Authored-By: Claude Sonnet 4.6 <[email protected]>

crates/clawhdf5-io/Cargo.toml

@@ -17,12 +17,15 @@ tokio = { version = "1", features = ["fs", "io-util"], optional = true }
 reqwest = { version = "0.12", features = ["json"], optional = true }
 serde = { version = "1", features = ["derive"], optional = true }
 serde_json = { version = "1", optional = true }
+mpi = { version = "0.8", optional = true }
 
 [dev-dependencies]
 tokio = { version = "1", features = ["full"] }
+tempfile = "3"
 
 [features]
 default = []
 mmap = ["memmap2", "libc"]
 async = ["tokio"]
 hsds = ["reqwest", "serde", "serde_json", "async"]
+mpi-io = ["mpi"]