Move the active-stage check and pledged-size update behind a scalar Rust ABI
leaf while keeping the CCtx layout and public C entry point in C. Preserve the
stage_wrong error encoding and the unsigned wrap behavior at U64::MAX.
Test Plan:
- cargo test --manifest-path rust/Cargo.toml --lib zstd_compress::tests (80 passed)
- cargo clippy --manifest-path rust/Cargo.toml --lib -- -D warnings
- rustfmt +nightly --edition 2021 rust/src/zstd_compress.rs --check
- make -C programs -j2 zstd
- git diff --check
ddidderr
a4a64f2e38