[akshitkhokhani]

Stream writers currently discard the byte count returned by their destination's write(). A destination accepting a short write silently loses the rest of the output, even though the outer writer reports consuming the input.

Retry the unwritten suffix in the C, CFFI and Rust compression/decompression writers, including compression flush. Preserve legacy callback destinations that return None after consuming the whole chunk. Zero progress raises BlockingIOError; invalid counts are rejected, and destination exceptions propagate. The documentation states this contract and excludes general nonblocking or post-failure resumability guarantees. copy_stream() is unchanged.

Fixes #370.

Validation on macOS ARM64 / CPython 3.13.7, using source-built backends and synthetic local data with network access disabled:

• Before the fix: the two short-write regressions fail on both C and CFFI; two legacy None controls pass on each.
• After the fix: both complete stream-writer test modules pass on C, CFFI and Rust — 45 tests plus 12 subtests per backend. Coverage includes compression write/flush, decompression, zero/negative/oversized/noninteger counts, the index protocol, output accounting and exception identity.
• Independent agent review reran all three backends and additional mixed short-count/None and index-exception controls; no blocking findings remained.
• Repository-wide Ruff and git diff --check pass. Rust changes were formatted with rustfmt. The full project/slow test suites, other Python versions and other operating systems were not run locally.

AI assistance was used for implementation, tests and independent review; no maintainer approval is implied.