feat(conformance): in-repo, reproducible conformance sweep

conformance/run.sh fetches eight public HDF5 corpora pinned by commit
(conformance/corpus.txt) into a gitignored cache, reads every file with
clawhdf5 (conformance/probe, a crate outside the workspace) and with
h5py/libhdf5 (ref.py), and the CVE files with h5dump, each under a timeout
and an address-space limit; compare.py classifies the files, report.py
writes CONFORMANCE.md and check.py gates on panics/hangs/crashes/OOM and on
regressions against conformance/baseline.json. ~25 s once cached.

Changes from the ad-hoc audit harness:
- the probe compares non-IEEE-layout floats (N-Bit) and integers with a bit
  offset or reduced precision as the values libhdf5 converts them to, not
  raw file bytes: 8 files that showed as mismatches now read identically;
- ref.py exits without tearing down h5py objects: libhdf5 2.0 aborts while
  freeing them for two files about half the time, which flipped them
  between ok and h5py-cannot-read from run to run;
- the file list is defined (list_files.py): netCDF classic files are left
  out, 11 HDF5 files the ad-hoc sweep missed are in.

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

conformance/fetch-corpus.sh

@@ -0,0 +1,39 @@
+#!/usr/bin/env bash
+# fetch-corpus.sh [cache_dir]
+#
+# Download the corpora pinned in conformance/corpus.txt into the (gitignored)
+# cache: <cache>/src/<name> is a shallow, sparse, blob-filtered checkout of the
+# pinned commit and <cache>/corpus/<name> links to the swept root inside it.
+# A corpus already checked out at its pinned commit is left alone, so a second
+# run costs nothing and needs no network.
+set -euo pipefail
+HERE="$(cd "$(dirname "$0")" && pwd)"
+CACHE="${1:-${CONFORMANCE_CACHE:-$HERE/.cache}}"
+mkdir -p "$CACHE/src" "$CACHE/corpus"
+CACHE="$(cd "$CACHE" && pwd)"
+
+retry() { local i; for i in 1 2 3 4; do "$@" && return 0; sleep $((i * 5)); done; return 1; }
+
+grep -v '^[[:space:]]*\(#\|$\)' "$HERE/corpus.txt" | while read -r name url commit root patterns; do
+  src="$CACHE/src/$name"
+  if [ -d "$src/.git" ] && [ "$(git -C "$src" rev-parse HEAD 2>/dev/null)" = "$commit" ]; then
+    echo "cached   $name @ ${commit:0:12}"
+  else
+    echo "fetching $name @ ${commit:0:12} from $url"
+    rm -rf "$src"
+    git init -q "$src"
+    git -C "$src" remote add origin "$url"
+    git -C "$src" config advice.detachedHead false
+    if [ -n "$patterns" ]; then
+      git -C "$src" config core.sparseCheckout true
+      # no-cone patterns (globs); `set -f` keeps the shell from expanding them
+      (set -f; printf '%s\n' $patterns) > "$src/.git/info/sparse-checkout"
+    fi
+    retry git -C "$src" fetch -q --depth 1 --filter=blob:none origin "$commit"
+    retry git -C "$src" checkout -q FETCH_HEAD
+    got="$(git -C "$src" rev-parse HEAD)"
+    [ "$got" = "$commit" ] || { echo "error: $name checked out $got, expected $commit" >&2; exit 1; }
+  fi
+  ln -sfn "$src/$root" "$CACHE/corpus/$name"
+done
+echo "corpus ready in $CACHE/corpus"