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]>

crates/clawhdf5-format/src/vl_data.rs

@@ -9,6 +9,7 @@ use alloc::{collections::BTreeMap, format, string::String, vec, vec::Vec};
 #[cfg(feature = "std")]
 use std::collections::BTreeMap;
 
+use crate::addr::to_usize;
 use crate::error::FormatError;
 use crate::global_heap::{GlobalHeapCollection, GlobalHeapIndex};
 
@@ -55,7 +56,7 @@ pub fn parse_vl_references(
 ) -> Result<Vec<VlElement>, FormatError> {
     let elem_size = 4 + offset_size as usize + 4; // length + address + index
     let total =
-        (num_elements as usize)
+        to_usize(num_elements)?
             .checked_mul(elem_size)
             .ok_or(FormatError::UnexpectedEof {
                 expected: usize::MAX,
@@ -68,7 +69,7 @@ pub fn parse_vl_references(
         });
     }
 
-    let mut elements = Vec::with_capacity(num_elements as usize);
+    let mut elements = Vec::with_capacity(to_usize(num_elements)?);
     let mut pos = 0;
 
     for _ in 0..num_elements {
@@ -406,7 +407,7 @@ impl<'a> VlResolver<'a> {
             let index =
                 GlobalHeapCollection::parse_index(self.file_data, offset, self.length_size)?;
             // parse_index checked that the collection lies in the file.
-            let end = offset + index.collection_size as usize;
+            let end = offset + to_usize(index.collection_size)?;
             self.check_overlap(offset, end)?;
             let coll = CachedCollection::new(index);
             if self.cached_bytes.saturating_add(coll.cost()) > self.budget {