Move block-level external sequence producer invocation, result
post-processing, source-length validation, and transfer ordering into a
Rust-owned ABI leaf. Retain the private CCtx transfer callback and C-side
fallback/block-compressor selection, preserving producer-error fallback
eligibility and direct invalid-sequence/transfer failures.
Test Plan:
- cargo test --manifest-path rust/Cargo.toml --lib
- cargo clippy --manifest-path rust/Cargo.toml --all-targets -- -D warnings
- make -B -C programs -j1 zstd
- make -C tests -j1 test-zstream ZSTREAM_TESTTIME=-T1s
- focused external_sequence_producer unit tests
ddidderr
9a68253710