feat: write dense group link storage (fractal heap + v2 B-tree)

A group with more than 8 links (libhdf5's compact max_compact default) is now
written densely instead of as inline Link messages: the links live in a
single-direct-block fractal heap indexed by a v2 B-tree of type 5 (link-name
index), referenced from the group's LinkInfo message. This matches libhdf5's
compact->dense switchover and keeps large groups out of the object header.

- Extract the byte-identical fractal-heap builder shared by dense attributes
  and dense links, parameterized by heap_id_length / max_heap_size. Attributes
  keep 8 / 40; links use 7 / 32 to match libhdf5 (reverse-engineered: an
  h5py-written dense group uses heap_id_length 7, max_heap_size 32, type-5
  record = hash(4) + heap_id(7) = 11 bytes). This was the cause of an initial
  "object overruns end of direct block" error from h5py.
- build_group_oh gained an optional dense LinkInfo (omitting inline Link
  messages); the two-pass file assembly allocates each group's link blob after
  its object header and rebuilds it with real target addresses in the final
  pass (link-message size is address-independent, so layout is stable).

Validated end-to-end: our reader round-trips dense groups, h5py reads the
groups we write, and dense attributes remain byte-identical (still h5py-valid).
The agent's 9-dataset memory group now writes densely and round-trips. Single
direct block only (~a couple thousand links); indirect blocks remain a TODO.

Tests: facade round-trip (20-link dense group + compact sibling) and an
h5py-gated interop test confirming libhdf5 reads our dense groups.

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

crates/clawhdf5/tests/h5py_interop_tests.rs

@@ -481,3 +481,41 @@ fn serde_json_minimal_parse(s: &str) -> Vec<f64> {
         .map(|v| v.trim().parse::<f64>().unwrap())
         .collect()
 }
+
+// ---------------------------------------------------------------------------
+// A_dense. Write a group with many links (dense storage) -> h5py reads all
+// ---------------------------------------------------------------------------
+
+#[test]
+fn clawhdf5_writes_dense_group_h5py_reads() {
+    skip_if_no_python!();
+    let dir = tempfile::tempdir().unwrap();
+    let path = dir.path().join("dense_group.h5");
+    let path_str = path.display().to_string();
+
+    // 20 links exceeds the compact threshold (8) -> dense fractal-heap storage.
+    let mut b = FileBuilder::new();
+    let mut g = b.create_group("big");
+    for i in 0..20 {
+        g.create_dataset(&format!("dataset_{i:03}"))
+            .with_i32_data(&[i, i * 10]);
+    }
+    b.add_group(g.finish());
+    b.write(&path).unwrap();
+
+    let script = format!(
+        r#"
+import h5py
+with h5py.File("{path_str}", "r") as f:
+    big = f["big"]
+    names = sorted(big.keys())
+    assert len(names) == 20, f"expected 20 links, got {{len(names)}}"
+    for i in range(20):
+        v = big[f"dataset_{{i:03}}"][()].tolist()
+        assert v == [i, i*10], f"link {{i}} = {{v}}"
+print("OK")
+"#
+    );
+    let out = run_python_output(&script);
+    assert_eq!(out, "OK");
+}