feat: add h5bench-equivalent Criterion benchmarks to clawhdf5-bench

Adds three Criterion benchmark suites mirroring the h5bench HPC I/O
benchmark workloads in pure Rust — no C libhdf5 required for the default
path, with an optional `libhdf5-compare` feature for side-by-side numbers.

  - benches/h5bench_write.rs: write_1d_contiguous, write_2d_chunked,
    write_f64_batch, write_multi_dataset, write_with_attrs
  - benches/h5bench_read.rs: read_sequential, read_f64_sequential,
    read_chunked_2d, read_from_disk, read_hyperslab
  - benches/h5bench_meta.rs: metadata_attrs_write, metadata_attrs_read,
    metadata_groups_create, metadata_groups_traverse, metadata_string_attrs

All benchmarks pass `cargo bench --bench <name> -- --test` and clippy
reports zero warnings.  Run with `cargo bench -p clawhdf5-bench`.

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

crates/clawhdf5-bench/Cargo.toml

@@ -25,8 +25,35 @@ path = "src/bin/consolidation_efficiency.rs"
 name = "ephemeral_perf"
 path = "src/bin/ephemeral_perf.rs"
 
+# ---------------------------------------------------------------------------
+# h5bench-equivalent Criterion benchmarks
+# ---------------------------------------------------------------------------
+
+[[bench]]
+name = "h5bench_write"
+harness = false
+
+[[bench]]
+name = "h5bench_read"
+harness = false
+
+[[bench]]
+name = "h5bench_meta"
+harness = false
+
 [dependencies]
 clawhdf5-agent = { path = "../clawhdf5-agent" }
 serde = { version = "1", features = ["derive"] }
 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 }
+
+[dev-dependencies]
+clawhdf5 = { path = "../clawhdf5" }
+criterion = { version = "0.5", features = ["html_reports"] }
+
+[features]
+# When enabled, benchmarks add matching libhdf5 variants for side-by-side comparison.
+libhdf5-compare = ["hdf5"]