test: regression for scale-offset float E-scale (raw + masked filter)

The HDF5 library does not implement the scale-offset filter's floating-point
E-scale mode. When asked for it (cd_values[0] = 1) it stores the chunk raw
(no minbits/minval header) and sets the chunk filter mask to skip the filter,
so such datasets read back verbatim purely by honoring the per-chunk filter
mask — no E-scale decoder is required.

Add a fixture written via the HDF5 low-level API (exact-representable values)
and a test asserting it reads back verbatim, locking in the filter-mask path.

Co-Authored-By: Claude Opus 4.8 <[email protected]>

CHANGELOG.md

@@ -39,6 +39,14 @@
   error. The previous `parse_vds_mappings` used a guessed layout that did not
   match real files and is replaced.
 
+### Tests
+- `clawhdf5-format`: regression test for **scale-offset float E-scale**
+  datasets. The HDF5 library does not implement E-scale encoding — when asked
+  for it (`cd_values[0] = 1`) it stores the chunk raw and sets the chunk filter
+  mask to skip the filter — so these files read back verbatim purely by
+  honoring the per-chunk filter mask. The test locks in that behavior against a
+  fixture produced via the HDF5 low-level API; no E-scale decoder is needed.
+
 ### Bug Fixes
 - `clawhdf5-format`: read **paged Fixed Array** chunk indexes. A filtered,
   fixed-dimension dataset with more than one data-block page (>1024 chunks by

crates/clawhdf5-format/tests/integration_test.rs

@@ -695,6 +695,20 @@ fn v4_virtual_dataset_2d_same_file_read() {
     );
 }
 
+#[test]
+fn scaleoffset_float_escale_reads_as_raw() {
+    // The scale-offset filter's floating-point *E-scale* mode (cd_values[0] = 1)
+    // is not actually implemented by the HDF5 library: when asked for it, HDF5
+    // stores the chunk raw (no minbits/minval header) and sets the chunk filter
+    // mask to skip the filter. So such a dataset must read back verbatim purely
+    // by honoring the per-chunk filter mask — no E-scale decoder is needed.
+    let file_data = include_bytes!("fixtures/scaleoffset_float_escale.h5");
+    let (raw, datatype, _) = read_chunked_dataset(file_data, "x");
+    let values = read_as_f64(&raw, &datatype).unwrap();
+    let expect: Vec<f64> = (0..20).map(|i| i as f64 * 0.25).collect();
+    assert_eq!(values, expect, "E-scale (raw + masked filter) must read verbatim");
+}
+
 #[test]
 fn v4_virtual_dataset_external_file_read() {
     use clawhdf5_format::data_read::read_raw_data_full_with_resolver;