perf(py): read an index list one group of chunks at a time

Each run of consecutive indices was its own uncached hyperslab read, so
a list over a compressed chunked dataset decoded the same chunk once per
run (d[range(0, 200000, 40)] over 20 gzip chunks: 8 s, h5py 0.014 s).
Plan::reads now groups the indices — a group ends only where a whole
chunk holds no selected index, or, unchunked, at a gap over 64 KiB — and
the selected rows are gathered from each group's block in Rust. Now
3.8 ms (h5py 4.1 ms, release, tank). The new test (1-D, 2-D and
contiguous, compared with h5py, 2 s bound) took 5.8 s before.

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

CHANGELOG.md

@@ -26,8 +26,8 @@
   read the whole dataset and slice it in numpy, and knew six dtypes. Now
   integers (negative from the end), slices with positive steps, `...`, one
   increasing list of integers per key and compound field names map onto the
-  facade's hyperslab selection (a list becomes one hyperslab per run of
-  consecutive indices), with h5py's results (numpy scalar for an all-integer
+  facade's hyperslab selection (a list is read one group of neighbouring
+  chunks at a time and picked from in memory), with h5py's results (numpy scalar for an all-integer
   key, 0-d array for `scalar[...]`) and h5py's errors for everything else
   (negative steps, `None`, boolean masks, out-of-range indices).
   `Dataset.dtype` is the numpy dtype h5py reports, for every integer and
@@ -69,6 +69,17 @@
   from the file (h5py's, zero for files it wrote) and stays zero-copy.
   The h5py comparisons now also compare every byte of structured values
   (`test_compound_padding_bytes_match_h5py` and `assert_same`).
+- **Index lists no longer decode the same chunks once per run.** A list
+  index was one uncached hyperslab read per run of consecutive indices, so
+  on a chunked, compressed dataset every run decoded its chunk again:
+  `d[list(range(0, 200000, 40))]` over 20 gzip chunks took 8 s (h5py:
+  0.014 s). The list is now read in groups — for a chunked dataset a group
+  ends only where a whole chunk holds no selected index, so each chunk is
+  decoded once; otherwise at a gap of more than 64 KiB — and the selected
+  rows are picked from each group in Rust. The same read now takes 3.8 ms
+  (h5py 4.1 ms; release build on tank, best of 5).
+  `test_a_long_index_list_decodes_each_chunk_once` compares 1-D, 2-D and
+  contiguous cases with h5py under a 2 s bound (5.8 s before, debug build).
 - **CI builds and tests the Python package.** It was excluded from CI.
   `scripts/ci-test.sh` now lints `clawhdf5-py`, builds the wheel with
   maturin, unpacks it under `target/` and runs the pytest suite; skipped