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/src/error.rs

@@ -6,7 +6,12 @@ use clawhdf5_format::error::FormatError;
 use clawhdf5_format::message_type::MessageType;
 
 /// Errors that can occur when using the high-level API.
+///
+/// Non-exhaustive: new failure modes add variants, so a `match` needs a
+/// wildcard arm. Failures of a [`Storage`](clawhdf5_format::storage::Storage)
+/// backend arrive as `Error::Format(FormatError::Storage(..))`.
 #[derive(Debug)]
+#[non_exhaustive]
 pub enum Error {
     /// I/O error from the filesystem.
     Io(std::io::Error),