perf(format): back large read buffers with transparent huge pages

A full read of a contiguous dataset is one memcpy from the mapped file,
yet ran at a quarter of h5py's speed on one thread: the fresh output Vec
took a page fault and a kernel page clear for every 4 KiB page written,
16384 per 64 MiB, costing several times the copy (the benchmark spent
6.2 s of 8 s in the kernel, 4.3M minor faults). numpy, so h5py, madvises
MADV_HUGEPAGE on allocations of 4 MiB or more; the typed readers' output,
the raw contiguous read and the chunk assembly buffer now do the same
(Linux only, libc as a Linux-only dependency; no-op otherwise).

New h5py comparison tests cover full and selection reads of contiguous
data for every 1-8-byte integer and float type, both byte orders, ranks
1-4, empty selections, and datasets past the 4 MiB threshold.

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

crates/clawhdf5-format/Cargo.toml

@@ -30,6 +30,10 @@ ruzstd = { version = "0.9", optional = true }
 bzip2 = { version = "0.6", optional = true }
 snap = { version = "1", optional = true }
 
+[target.'cfg(target_os = "linux")'.dependencies]
+# madvise(MADV_HUGEPAGE) for large read buffers (see src/bulk_alloc.rs).
+libc = { version = "0.2", default-features = false }
+
 [dev-dependencies]
 half = { workspace = true }
 serde_json = "1"