test: cover Fixed Array chunk indexes against real files

The Extensible Array bug reached a release because no fixture had more
chunks than fit inline, so its data blocks were never read. The Fixed
Array index had the same blind spot: nothing exercised it above a
handful of chunks, and nothing reached the paged layout at all.

Checked at 100, 5 000 and 200 000 chunks plus a sparse dataset that
leaves whole pages uninitialised. It is correct throughout — it does
keep its page-init bitmap inside the data block, which is the
difference from the Extensible Array that made assuming otherwise a
bug. Adding the tests so that stays true.

Co-Authored-By: Claude Opus 5 (1M context) <[email protected]>

CHANGELOG.md

@@ -27,6 +27,12 @@
   Covered now by interop tests at 4, 37, 400, 5 000 and 200 000 chunks (the
   last large enough for paged data blocks), plus sparse, gzip-filtered and
   2-D cases. Writing is unaffected; this is a read-path bug.
+- `clawhdf5-format`: the sibling Fixed Array index (fixed dimensions written
+  with `libver='latest'`) was checked against the same range and is correct,
+  including paged data blocks and sparse datasets — it really does keep its
+  page-init bitmap in the data block, where the Extensible Array does not.
+  It had no real-file coverage above the inline sizes either, so it now has
+  the same tests.
 
 ### Security
 - `clawhdf5-format`: **a crafted file could crash any reader through B-tree v2

crates/clawhdf5/tests/h5py_interop_tests.rs

@@ -1241,3 +1241,80 @@ with h5py.File("{two_d_str}", "w", libver="latest") as f:
         0
     );
 }
+
+#[test]
+fn h5py_fixed_array_chunk_index_clawhdf5_reads() {
+    // Fixed dimensions plus libver='latest' give a Fixed Array chunk index.
+    // Its data blocks are paged above 2^page_bits elements (1024 by default),
+    // and unlike the Extensible Array it keeps the page-init bitmap in the
+    // data block itself — a difference worth pinning down, since assuming
+    // otherwise is exactly what made the Extensible Array reader wrong. The
+    // sparse case leaves whole pages uninitialised so the bitmap is actually
+    // consulted rather than being all ones.
+    skip_if_no_python!();
+    let dir = tempfile::tempdir().unwrap();
+
+    for n in [100usize, 5_000, 200_000] {
+        let path = dir.path().join(format!("fa_{n}.h5"));
+        let path_str = path.display().to_string();
+        run_python(&format!(
+            r#"
+import h5py, numpy as np
+with h5py.File("{path_str}", "w", libver="latest") as f:
+    d = f.create_dataset("x", shape=({n},), chunks=(1,), dtype="i4")
+    d[...] = np.arange({n}, dtype="i4")
+"#
+        ));
+        let bytes = std::fs::read(&path).unwrap();
+        assert!(
+            bytes.windows(4).any(|w| w == b"FAHD"),
+            "n={n}: fixture is not indexed by a Fixed Array"
+        );
+        let values = File::open(&path)
+            .unwrap()
+            .dataset("x")
+            .unwrap()
+            .read_i32()
+            .unwrap();
+        assert_eq!(values.len(), n, "n={n}");
+        let wrong = values
+            .iter()
+            .enumerate()
+            .filter(|&(i, &v)| v != i as i32)
+            .count();
+        assert_eq!(wrong, 0, "n={n}: {wrong} elements read back wrong");
+    }
+
+    let sparse = dir.path().join("fa_sparse.h5");
+    let sparse_str = sparse.display().to_string();
+    let (n, step) = (200_000usize, 997usize);
+    run_python(&format!(
+        r#"
+import h5py
+with h5py.File("{sparse_str}", "w", libver="latest") as f:
+    d = f.create_dataset("x", shape=({n},), chunks=(1,), dtype="i4", fillvalue=-1)
+    for i in list(range(0, {n}, {step})) + list(range(0, 40)):
+        d[i] = i
+"#
+    ));
+    let values = File::open(&sparse)
+        .unwrap()
+        .dataset("x")
+        .unwrap()
+        .read_i32()
+        .unwrap();
+    assert_eq!(values.len(), n);
+    let wrong = values
+        .iter()
+        .enumerate()
+        .filter(|&(i, &v)| {
+            let expected = if i % step == 0 || i < 40 {
+                i as i32
+            } else {
+                -1
+            };
+            v != expected
+        })
+        .count();
+    assert_eq!(wrong, 0, "sparse: {wrong} of {n} elements read back wrong");
+}