The CLI already delegated one zstd frame at a time to Rust, but the C
FIO_decompressFrames loop still owned repeated-frame dispatch. That left
concatenated zstd streams split across the language boundary and made the
Rust frame helper unable to preserve the next mixed-format header itself.
Add a Rust multi-frame adapter that repeatedly probes four buffered bytes,
uses the existing one-frame decoder, accumulates decoded output and progress,
and leaves a following non-zstd header or short trailing input untouched for
C's format dispatcher. Keep C responsible for probing the first format,
non-zstd dispatch, diagnostics, cleanup, and final file accounting. Decoder
errors still preserve the current input for FIO_zstdErrorHelp().
Test Plan:
- `cargo test --manifest-path rust/Cargo.toml --lib fileio_asyncio -- --test-threads=1` -- 20 passed
- `cargo clippy --manifest-path rust/Cargo.toml --lib -- -D warnings` -- passed
- `cargo +nightly fmt --manifest-path rust/Cargo.toml -- --check` -- passed
- `make -B -C tests -j2 test-cli-tests` -- all 41 passed
- `git diff --cached --check` -- passed
- Full root `cargo clippy --all-targets/--benches/--tests -- -D warnings` remains blocked by the pre-existing `manual_repeat_n` warning in `rust/src/zstd_compress.rs` test code; no unrelated change was made.