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/extensible_array.rs

@@ -9,6 +9,7 @@ extern crate alloc;
 #[cfg(not(feature = "std"))]
 use alloc::{format, vec, vec::Vec};
 
+use crate::addr::to_usize;
 use crate::chunk_grid::ChunkGrid;
 use crate::chunked_read::ChunkInfo;
 use crate::error::FormatError;
@@ -451,7 +452,7 @@ pub fn read_extensible_array_chunks(
     // Parse index block (EAIB): signature(4) + version(1) + client_id(1)
     // + header address(offset_size), then the inline elements, then the
     // direct data block addresses, then the super block addresses.
-    let ib_offset = header.index_block_address as usize;
+    let ib_offset = to_usize(header.index_block_address)?;
     let ib_header_size = 4 + 1 + 1 + os;
     ensure_len(file_data, ib_offset, ib_header_size)?;
 
@@ -463,7 +464,7 @@ pub fn read_extensible_array_chunks(
     let mut pos = ib_offset + ib_header_size;
 
     let mut chunks = Vec::new();
-    let total_elements = header.num_elements as usize;
+    let total_elements = to_usize(header.num_elements)?;
 
     let dmin = header.min_dblk_nelmts as usize;
     if dmin == 0 || !dmin.is_power_of_two() {
@@ -563,7 +564,7 @@ pub fn read_extensible_array_chunks(
             }
             chunks.extend(read_data_block_elements(
                 file_data,
-                addr as usize,
+                to_usize(addr)?,
                 dblk_nelmts,
                 header,
                 offset_size,
@@ -592,7 +593,7 @@ pub fn read_extensible_array_chunks(
         if !is_undefined_addr(sb_addr, offset_size) {
             chunks.extend(read_super_block(
                 file_data,
-                sb_addr as usize,
+                to_usize(sb_addr)?,
                 ndblks,
                 dblk_nelmts,
                 header,
@@ -676,7 +677,7 @@ fn read_super_block(
         if !is_undefined_addr(addr, offset_size) {
             chunks.extend(read_data_block_elements(
                 file_data,
-                addr as usize,
+                to_usize(addr)?,
                 dblk_nelmts,
                 header,
                 offset_size,