fix(mt): gate production-only sequence projection

Keep the MT sequence-state projection helper available to the C ABI
orchestration while excluding it from the standalone Rust test target,
where the production entry point is intentionally not compiled. This
keeps the capped Rust test check warning-free without changing runtime
behavior or the C layout contract.

Test Plan:
- ulimit -v 41943040; CARGO_BUILD_JOBS=1 cargo check --manifest-path rust/Cargo.toml --tests
- git diff --check

rust/src/zstdmt_compress.rs

@@ -543,6 +543,7 @@ struct ZSTDMT_compressionJobSequenceProjection {
 }
 
 #[inline]
+#[cfg(not(test))]
 unsafe fn compression_job_sequence_projection(
     state: *const ZSTDMT_compressionJobSequenceState,
 ) -> Option<ZSTDMT_compressionJobSequenceProjection> {