Project the scalar ZSTD_compressStream2 end directive into Rust and keep C responsible for the private streaming context, existing parameter error, and diagnostic text. The Rust range policy preserves the original unsigned enum validation for continue, flush, and end.
Test Plan: ulimit -v 41943040; CARGO_BUILD_JOBS=1 cargo clippy --manifest-path rust/Cargo.toml --all-targets -- -D warnings; cargo +nightly fmt --manifest-path rust/Cargo.toml --all -- --check; make -j1; ./tests/rustLibSmoke; make -j1 -C tests test (all shell tests, large streaming tests, native tester, fuzzer phases, and zstream tester passed).