Write files HDF5 1.8 can read: FileWriter/FileBuilder::libver_bounds

New `LibVer` (V18, V110, V112, V114, V200, Latest) and
`libver_bounds(low, high)` on the format crate's `FileWriter` and the
facade's `FileBuilder`, as libhdf5's H5Pset_libver_bounds / h5py's
libver=(low, high). The default stays (V110, Latest), byte for byte what
was written before.

With a low bound of 1.8: superblock version 2, layout message version 3
(contiguous, compact, chunked) and a version-1 B-tree chunk index for
every chunked dataset, resizable ones included -- what libhdf5 2.x writes
under libver=('v108', 'latest'). The new chunk B-tree writer
(btree_v1_write.rs) replays H5B_insert with the H5Dbtree.c callbacks for
row-major insertion (split ratios 0.1/0.5/0.9, right keys moved as
H5D__btree_cmp3 moves them, root kept in place): its trees equal
libhdf5's node for node for 1-D/2-D/3-D, 2- and 3-level, filtered and
unfiltered datasets (libhdf5 writing without a chunk cache).

The high bound refuses, with FormatError::LibverBound before anything is
written, what needs a newer format: virtual datasets and the paged
file-space strategy (1.10), the 1.12 reference types (datatype v4),
native complex (datatype v5, HDF5 2.0), and a low bound above the high.

Tests: tools/tests/libver_v18.rs writes every writer feature under
(V18, V18), and HDF5 1.8.23's h5dump (scripts/build-hdf5-1.8.sh; skipped
when absent) dumps it exactly as h5dump 1.14 does and returns our bytes
for every numeric dataset; h5py, clawhdf5 and h5rs check --data agree;
then FileEditor grows/appends/annotates it and h5py appends, and every
reader checks again. read_harness gains --v18 and --chunk N.

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

scripts/build-hdf5-1.8.sh

@@ -0,0 +1,43 @@
+#!/usr/bin/env bash
+# Build libhdf5 1.8.23 (the last 1.8 release) with its command-line tools, as
+# the oracle for files written with `LibVer::V18` (the `libver_v18` test in
+# clawhdf5-tools finds h5dump through CLAWHDF5_H5DUMP18 or this default prefix).
+#
+#   bash scripts/build-hdf5-1.8.sh [PREFIX]
+#
+# PREFIX defaults to ~/.cache/hdf5-1.8.23; sources go to PREFIX-src and the
+# build tree to PREFIX-build. Reuses an existing install. Needs git, cmake, a C
+# compiler and zlib headers.
+set -euo pipefail
+PREFIX="${1:-$HOME/.cache/hdf5-1.8.23}"
+SRC="$PREFIX-src"
+BUILD="$PREFIX-build"
+if [ -x "$PREFIX/bin/h5dump" ]; then
+    echo "reusing $PREFIX/bin/h5dump"
+    "$PREFIX/bin/h5dump" --version
+    exit 0
+fi
+if [ ! -d "$SRC" ]; then
+    git clone --depth 1 --branch hdf5-1_8_23 https://github.com/HDFGroup/hdf5.git "$SRC"
+fi
+# 1.8 predates current compilers (GCC 14 turns its pointer-type mismatches in
+# the tools into errors): pin gnu99 and demote those errors to warnings.
+CFLAGS18="-w -std=gnu99 -Wno-error=incompatible-pointer-types"
+CFLAGS18="$CFLAGS18 -Wno-error=implicit-function-declaration -Wno-error=int-conversion"
+cmake -S "$SRC" -B "$BUILD" \
+    -DCMAKE_BUILD_TYPE=Release \
+    -DCMAKE_INSTALL_PREFIX="$PREFIX" \
+    -DCMAKE_C_FLAGS="$CFLAGS18" \
+    -DBUILD_SHARED_LIBS=ON \
+    -DBUILD_TESTING=OFF \
+    -DHDF5_BUILD_TOOLS=ON \
+    -DHDF5_BUILD_EXAMPLES=OFF \
+    -DHDF5_BUILD_CPP_LIB=OFF \
+    -DHDF5_BUILD_FORTRAN=OFF \
+    -DHDF5_BUILD_HL_LIB=OFF \
+    -DHDF5_BUILD_JAVA=OFF \
+    -DHDF5_ENABLE_Z_LIB_SUPPORT=ON \
+    -DHDF5_ENABLE_SZIP_SUPPORT=OFF
+cmake --build "$BUILD" -j "${JOBS:-6}"
+cmake --install "$BUILD"
+"$PREFIX/bin/h5dump" --version