ZSTD_compressStream2_c still owns private context mutation, diagnostics, buffer updates, and codec initialization, but its stable-input transparent-initialization decision was a remaining scalar policy island in C. Move the continuation validation, block-size threshold, format-specific progress hint, and wrapping size arithmetic behind a Rust projection. Preserve the original C error messages and return values while keeping all private context state on the C side.\n\nThe bridge uses explicit ABI layout assertions for the mixed pointer/size projection. Focused Rust tests cover buffered and non-continue calls, block-boundary initialization, empty and short stable input, both frame formats, matching continuation state, invalid source/position, and null bridge input.\n\nTest Plan:\n- ulimit -v 41943040; CARGO_BUILD_JOBS=1 cargo +nightly fmt --manifest-path rust/Cargo.toml --all -- --check\n- ulimit -v 41943040; CARGO_BUILD_JOBS=1 cargo clippy --manifest-path rust/Cargo.toml --all-targets -- -D warnings\n- ulimit -v 41943040; CARGO_BUILD_JOBS=1 cargo clippy --manifest-path rust/cli/Cargo.toml --all-targets -- -D warnings\n- Full capped native and original-test verification follows after the batch is committed.