docs+fix: add h5bench benchmark results and repair libhdf5-compare feature

Add h5bench-equivalent Criterion benchmark results to BENCHMARKS.md
(sequential read/write, chunked read/write, metadata throughput).

Fix libhdf5-compare feature for HDF5 1.14.x:
- Switch to hdf5-metno 0.12 (aliased as 'hdf5') in clawhdf5-bench
- Fix h5bench_meta.rs: AttributeBuilderEmpty::create takes &str not &String;
  shape=[1] dataset uses write(&[val]) not write_scalar
- Fix h5bench_read.rs: libhdf5-compare variant now writes its own reference
  file via hdf5-metno instead of dumping clawhdf5 bytes (avoids float
  datatype message incompatibility)

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

crates/clawhdf5-bench/Cargo.toml

@@ -55,7 +55,8 @@ serde_json = "1"
 tempfile = "3"
 # Optional: libhdf5 C wrapper for side-by-side comparison (requires system libhdf5).
 # Enable with: cargo bench -p clawhdf5-bench --features libhdf5-compare
-hdf5 = { version = "0.8", optional = true }
+# Uses hdf5-metno (fork of hdf5 crate) which supports HDF5 1.14.x.
+hdf5 = { version = "0.12", optional = true, package = "hdf5-metno" }
 
 [dev-dependencies]
 clawhdf5 = { path = "../clawhdf5" }