fix(format): record the content size in zstd frames

Filter 32015 chunks were written with the streaming encoder
(zstd::encode_all), whose frames carry no content size. The registered
HDF5 Zstandard filter (H5Zzstd.c, libhdf5 + hdf5plugin) sizes its output
from ZSTD_getFrameContentSize and fails on such frames, so h5py could not
read our zstd datasets ("filter returned failure during read"). Compress
with the one-shot API, which records the size.

Tests: zstd_frames_record_content_size (content size was None before),
hdf5plugin_reads_our_zstd (ignored interop test; failed before).

Co-Authored-By: Claude Opus 5.5 (1M context) <[email protected]>

crates/clawhdf5-format/tests/writer_h5py_tests.rs

@@ -985,3 +985,16 @@ fn hdf5plugin_reads_our_lz4() {
     });
     assert_eq!(got, data);
 }
+
+/// libhdf5's Zstandard plugin must decode what we write (it could not while
+/// our frames lacked the content size).
+#[cfg(feature = "zstd")]
+#[test]
+#[ignore = "requires Python h5py + hdf5plugin"]
+fn hdf5plugin_reads_our_zstd() {
+    let data: Vec<f64> = (0..1000).map(|i| (i % 37) as f64 * 0.5).collect();
+    let got = hdf5plugin_roundtrip("zstd", &data, |ds| {
+        ds.with_zstd(3);
+    });
+    assert_eq!(got, data);
+}