format: add the Storage trait; make the error enums non-exhaustive

Range-read milestone M1, first step (docs/design/range-reads.md §3(a)):
a synchronous, no_std read interface with u64 offsets, read_at returning
Cow<[u8]>, read_ranges, len and an as_contiguous fast path. Implemented
for [u8], Vec<u8>, &T, Box<T> and Arc<T>; slices serve borrowed bytes.
read_exact_at reproduces the parsers' UnexpectedEof bounds error exactly,
so converted modules keep their error values.

FormatError gains Storage(String) and ContiguousStorageRequired; it and
the facade Error are now #[non_exhaustive] (breaking for exhaustive
matches, noted in the changelog; the Python bindings' match gets a
wildcard arm).

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

crates/clawhdf5-py/src/lib.rs

@@ -82,6 +82,7 @@ pub(crate) fn to_py_err(e: clawhdf5_rs::Error) -> PyErr {
         | Error::ZeroCopyUnaligned { .. } => {
             PyErr::new::<pyo3::exceptions::PyValueError, _>(e.to_string())
         }
+        _ => PyErr::new::<pyo3::exceptions::PyOSError, _>(e.to_string()),
     }
 }