fix(read): decode on the calling thread when rayon's pool has one thread

Full reads of chunked datasets handed their chunks to rayon. With a
one-thread pool (concurrent_read --decode-threads 1, RAYON_NUM_THREADS=1)
every thread reading through a File queued behind that single worker, so
16 readers decoded on one core: per-thread CPU time showed one thread
doing all the decoding and the readers almost none, and full reads
stopped at about 2x one thread. The cached full-read path and the
uncached reader behind verify_provenance now decode inline when the pool
cannot parallelise (parallel_read::pool_can_parallelise).

The File's chunk cache was the suspect but not the cause: datasets over
its budget were already read without inserting, and skipping its lookups
gained only a few percent at 16 threads.

The regression test keeps a one-thread global pool's worker busy and
requires a full read and verify_provenance to finish anyway; before the
fix both waited for the worker (timed out).

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

CHANGELOG.md

@@ -2,6 +2,19 @@
 
 ## Unreleased
 
+### Concurrent reads (2026-09-26)
+- **Full reads of chunked datasets scale with threads again when rayon's
+  pool has one thread.** Each full read handed its chunks to rayon to
+  decode; with a one-thread pool (`RAYON_NUM_THREADS=1`, or
+  `concurrent_read --decode-threads 1`) every thread reading through a
+  `File` queued behind that single worker, so N readers decoded on one core
+  and throughput stopped at about 2x one thread. Such reads, and
+  `verify_provenance`'s uncached reader, now decode on the calling thread
+  (`clawhdf5_format::parallel_read::pool_can_parallelise`). The `File`'s
+  chunk cache, the suspect in `docs/known-issues.md`, was not the cause:
+  reads of datasets larger than its budget already skipped inserting, and
+  its lookups cost a few percent at 16 threads.
+
 ### Plugin filters (2026-09-26)
 - **LZF, bitshuffle, bzip2 and Blosc read and write, in pure Rust.** Files
   written by h5py with `compression="lzf"`, or with hdf5plugin's