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/local_heap.rs

@@ -3,6 +3,7 @@
 #[cfg(not(feature = "std"))]
 use alloc::string::String;
 
+use crate::addr::to_usize;
 use crate::error::FormatError;
 
 /// Parsed HDF5 Local Heap header.
@@ -140,15 +141,15 @@ impl LocalHeap {
 
     /// Read a null-terminated string from the heap's data segment at the given byte offset.
     pub fn read_string(&self, file_data: &[u8], string_offset: u64) -> Result<String, FormatError> {
-        let seg_addr = self.data_segment_address as usize;
+        let seg_addr = to_usize(self.data_segment_address)?;
         let str_start =
             seg_addr
-                .checked_add(string_offset as usize)
+                .checked_add(to_usize(string_offset)?)
                 .ok_or(FormatError::Overflow(
                     "local heap seg_addr + string_offset overflow".into(),
                 ))?;
         let seg_end = seg_addr
-            .checked_add(self.data_segment_size as usize)
+            .checked_add(to_usize(self.data_segment_size)?)
             .ok_or(FormatError::Overflow(
                 "local heap seg_addr + data_segment_size overflow".into(),
             ))?;