format: no truncating u64 -> usize casts

Every `u64 as usize` cast in clawhdf5-format (115 on wasm32) now goes
through addr::to_usize for values read from the file — addresses, lengths,
counts, dimensions: FormatError::Overflow where the value does not fit
instead of wrapping onto another part of the file on a 32-bit target — or
addr::saturating_usize for counts bounded by something in memory (codec
progress counters, writer sizes), which fail a bounds check or allocation
rather than wrap. A chunk whose offset does not fit lies outside the
dataset and is skipped; partial reads treat such an offset as out of the
buffers. On 64-bit targets nothing changes.

scripts/check-32bit-casts.sh (run by ci-test.sh) lints the wasm32 build
with clippy's cast_possible_truncation and fails on any u64 -> usize
finding; before this commit it listed 115.

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

scripts/check-32bit-casts.sh

@@ -0,0 +1,35 @@
+#!/usr/bin/env bash
+# CI check: clawhdf5-format has no truncating `u64 as usize` cast on a 32-bit
+# target. HDF5 addresses and lengths are 64-bit; on wasm32 (or any 32-bit
+# target) such a cast silently wraps an address past 4 GiB onto another part
+# of the file. File values go through `addr::to_usize` (a clean error) and
+# in-memory counts through `addr::saturating_usize`.
+#
+# Lints wasm32-unknown-unknown with clippy's cast_possible_truncation and
+# fails on any u64 -> usize finding (other truncations are not checked here).
+#
+# Usage:
+#   ./scripts/check-32bit-casts.sh
+#
+# Prerequisites:
+#   rustup target add wasm32-unknown-unknown
+
+set -euo pipefail
+
+TARGET="wasm32-unknown-unknown"
+echo "==> Checking for truncating u64 -> usize casts in clawhdf5-format ($TARGET)"
+
+out=$(cargo clippy -p clawhdf5-format --target "$TARGET" \
+    --features plugin-filters --message-format short \
+    -- -A clippy::all -W clippy::cast_possible_truncation 2>&1) || {
+    echo "$out"
+    echo "==> clippy failed" >&2
+    exit 1
+}
+found=$(grep -F 'casting `u64` to `usize`' <<<"$out" || true)
+if [ -n "$found" ]; then
+    echo "$found"
+    echo "==> use addr::to_usize (file values) or addr::saturating_usize (in-memory counts)" >&2
+    exit 1
+fi
+echo "==> no truncating u64 -> usize casts"