docs: the SWMR follow loop stops for a writer that died, not only one that closed

The README example looped while swmr_writer_active(), which never ends when
the writer crashed or was killed: libhdf5 clears the SWMR-write flag only on
close (the mid-write fixture keeps it set for good). The loop now also stops
after a minute without growth, and the README, the swmr_writer_active docs
(with the same loop as a compiled no_run doctest) and the design say why.

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

README.md

@@ -518,17 +518,31 @@ races the writer (a checksum that fails mid-flush) is retried, up to 100
 attempts as in libhdf5, and never returned torn.
 
 ```rust
+use std::time::{Duration, Instant};
+
 let file = clawhdf5::File::open_swmr("live.h5")?;
 let mut ds = file.dataset("samples")?;
-while file.swmr_writer_active()? {
+let (mut seen, mut last_growth) = (0, Instant::now());
+// Stop when the writer closes the file, or when the dataset has not grown
+// for a minute: a writer that crashed or was killed never clears the
+// SWMR-write flag, so `swmr_writer_active()` alone can stay true forever.
+while file.swmr_writer_active()? && last_growth.elapsed() < Duration::from_secs(60) {
     ds.refresh()?;
     let n = ds.shape()?[0];
-    // read the new rows ...
-    std::thread::sleep(std::time::Duration::from_millis(100));
+    if n > seen {
+        // read rows seen..n ...
+        (seen, last_growth) = (n, Instant::now());
+    }
+    std::thread::sleep(Duration::from_millis(100));
 }
 ds.refresh()?; // the final extent
 ```
 
+`swmr_writer_active()` reads the superblock's SWMR-write flag, which libhdf5
+clears only when the writer closes the file; a file whose writer died keeps
+it set (as the mid-write copy in `tests/fixtures/swmr_mid_write.h5` does),
+so a follower needs its own stop condition, like the idle timeout above.
+
 Design and limits: [docs/design/swmr.md](docs/design/swmr.md).
 
 ### Python