ZSTDMT_updateCParams_whileCompressing already delegated parameter derivation to Rust, but C still owned the transition publication order and independently updated compressionLevel. Return a Rust-owned scalar result that carries the requested level with the derived compression parameters, while preserving the active frame window and all private MT context state in C.\n\nThe new projection and result have explicit C/Rust layout assertions. Focused Rust tests exercise unknown-size and explicit-size hints, LDM and override inputs, saved-window restoration, requested-level forwarding, and the exported ABI wrapper.\n\nTest Plan:\n- ulimit -v 41943040; CARGO_BUILD_JOBS=1 cargo fmt/clippy gates passed before staging\n- git diff --cached --check\n- Full capped native and original-test verification follows after the batch is committed.