format: read a contiguous selection's runs merged across small gaps

gather_storage merged only runs that touch, so a strided selection of a
contiguous dataset over a Storage became one range (and one owned Vec) per
element: a stride-2 read of 32M f32 through File::open_storage made
16,777,232 read_at calls, took 2.0 s and peaked at 2.09 GB.

The selection is now walked twice. The first walk checks the runs and plans
spans: runs in increasing order at most 4 KiB apart (GATHER_GAP_BYTES) are
read as one span up to 8 MiB (GATHER_SPAN_BYTES; a longer run is split), so
nothing is stored per run. The spans are fetched in RAW_BATCH_BYTES batches
while the second walk copies each run out of its span. Same checks and
errors as before.

The same read is now 32 reads and 0.31 s (File::open: 0.08 s).
contiguous_read_interop: every h5py-checked selection is also read through
File::open_storage and must give libhdf5's bytes; a new test bounds the
range reads of strided, blocked, column and point selections (stride 2: at
most 1 data read; 563,200 before).

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

crates/clawhdf5-format/src/storage.rs

@@ -420,7 +420,7 @@ pub(crate) enum Extent<'a> {
 impl<'a> ExtentBytes<'a> {
     /// Fetch `reqs`, extents `base..base + reqs.len()` of the read: the
     /// bytes of those wanted, and the bounds check of all of them.
-    fn fetch<S: Storage + ?Sized>(
+    pub(crate) fn fetch<S: Storage + ?Sized>(
         file: &'a S,
         reqs: &[ExtentReq],
         base: usize,