facade: open_swmr reads live only files with the SWMR-write flag

A file opened with open_swmr whose superblock does not have the SWMR-write
flag (its writer has closed it) is now read exactly as File::open reads it:
bounded by its recorded end of file, through the chunk cache, each
operation tried once, and is_swmr_read() is false. Before, every file opened
with open_swmr ignored its recorded end of file, so a closed file whose end
of file is below its length (h5clear_fsm_persist_less.h5, or the mid-write
fixture with its flags cleared) listed and read objects that File::open and
libhdf5's plain reader refuse. Opening such a file is not retried once the
superblock has been read.

libhdf5's SWMR reader is looser than either (it reads the fixture's chunk
index past the end of file even with the flags cleared); a test pins what
h5py does and docs/design/swmr.md says why we follow the plain reader.

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

CHANGELOG.md

@@ -27,6 +27,10 @@ Design: `docs/design/swmr.md`.
   would read as fill where the writer has since written). A file open for
   writing without SWMR is refused with `Error::Locked`, as libhdf5 refuses
   it.
+  Only a file whose superblock has the SWMR-write flag when it is opened is
+  read this way; any other file (its writer has closed it) reads exactly as
+  `File::open` reads it, bounded by its recorded end of file, and
+  `is_swmr_read()` is `false`.
 - **`Dataset::refresh()`** reads the dataset's object header again
   (`H5Drefresh`, h5py `Dataset.refresh()`), so `shape()` and later reads
   see the writer's appends; a handle keeps its extent until refreshed.