refactor(mt): move per-job parameter policy to Rust

Move the pure multithreaded worker-parameter policy into Rust: only the first
job keeps the frame checksum flag, worker jobs disable LDM, and every worker
context runs with zero nested workers. Keep the complete C-owned parameter
structure and codec initialization in the adapter, crossing the ABI with a
small layout-checked scalar projection.

Test Plan:
- rustfmt --edition 2021 --check rust/src/zstdmt_compress.rs
- ulimit -v 41943040; CARGO_BUILD_JOBS=1 cargo clippy --manifest-path rust/Cargo.toml --all-targets -- -D warnings
- ulimit -v 41943040; make -j1
- ulimit -v 41943040; CARGO_BUILD_JOBS=1 cargo test --manifest-path rust/Cargo.toml --all-targets
- ulimit -v 41943040; CARGO_BUILD_JOBS=1 cargo test --manifest-path rust/cli/Cargo.toml --all-targets
- ulimit -v 41943040; make -j1 -C tests test

rust/src/zstdmt_compress.rs

@@ -45,7 +45,6 @@ const ZSTD_BTOPT: c_int = 7;
 const ZSTD_BTULTRA: c_int = 8;
 const ZSTD_BTULTRA2: c_int = 9;
 const ZSTD_PS_ENABLE: c_int = 1;
-#[cfg(test)]
 const ZSTD_PS_DISABLE: c_int = 2;
 
 const RSYNC_LENGTH: usize = 32;
@@ -66,6 +65,57 @@ pub struct ZSTDMT_chunkProcessResult {
     pub lastBlockSize: usize,
 }
 
+/// Per-job parameter policy applied before a worker initializes its private
+/// compression context. The full `ZSTD_CCtx_params` remains C-owned; only the
+/// three scalar fields changed by the MT worker policy cross the ABI.
+#[repr(C)]
+#[derive(Clone, Copy, Debug, Default, PartialEq, Eq)]
+pub struct ZSTDMT_compressionJobParameters {
+    pub checksumFlag: c_int,
+    pub ldmEnable: c_int,
+    pub nbWorkers: c_uint,
+}
+
+const _: () = {
+    assert!(offset_of!(ZSTDMT_compressionJobParameters, checksumFlag) == 0);
+    assert!(offset_of!(ZSTDMT_compressionJobParameters, ldmEnable) == size_of::<c_int>());
+    assert!(offset_of!(ZSTDMT_compressionJobParameters, nbWorkers) == 2 * size_of::<c_int>());
+    assert!(
+        size_of::<ZSTDMT_compressionJobParameters>()
+            == 2 * size_of::<c_int>() + size_of::<c_uint>()
+    );
+};
+
+#[inline]
+fn prepare_compression_job_parameters(
+    job_id: c_uint,
+    checksum_flag: c_int,
+    _ldm_enable: c_int,
+    _nb_workers: c_uint,
+) -> ZSTDMT_compressionJobParameters {
+    ZSTDMT_compressionJobParameters {
+        /* The first job writes the frame checksum; later jobs have their
+         * checksum accounted for by the serial MT state. */
+        checksumFlag: if job_id == 0 { checksum_flag } else { 0 },
+        /* LDM sequences are generated by the serial state, not per worker. */
+        ldmEnable: ZSTD_PS_DISABLE,
+        /* Worker contexts are initialized as single-threaded contexts. */
+        nbWorkers: 0,
+    }
+}
+
+/// Apply the pure per-worker MT parameter policy while C retains the complete
+/// compression-parameter structure and all codec initialization.
+#[no_mangle]
+pub extern "C" fn ZSTDMT_rust_prepareCompressionJobParameters(
+    jobID: c_uint,
+    checksumFlag: c_int,
+    ldmEnable: c_int,
+    nbWorkers: c_uint,
+) -> ZSTDMT_compressionJobParameters {
+    prepare_compression_job_parameters(jobID, checksumFlag, ldmEnable, nbWorkers)
+}
+
 /// Scalar job state used by the Rust compression-job scheduler.
 ///
 /// The job descriptor, pools, synchronization, and codec state remain
@@ -7157,6 +7207,38 @@ mod tests {
         );
     }
 
+    #[test]
+    fn compression_job_parameters_disable_worker_owned_features() {
+        assert_eq!(
+            prepare_compression_job_parameters(0, 1, ZSTD_PS_ENABLE, 4),
+            ZSTDMT_compressionJobParameters {
+                checksumFlag: 1,
+                ldmEnable: ZSTD_PS_DISABLE,
+                nbWorkers: 0,
+            }
+        );
+    }
+
+    #[test]
+    fn compression_job_parameters_clear_checksum_after_first_job() {
+        assert_eq!(
+            prepare_compression_job_parameters(3, 1, ZSTD_PS_ENABLE, 4),
+            ZSTDMT_compressionJobParameters {
+                checksumFlag: 0,
+                ldmEnable: ZSTD_PS_DISABLE,
+                nbWorkers: 0,
+            }
+        );
+        assert_eq!(
+            prepare_compression_job_parameters(9, 0, ZSTD_PS_DISABLE, 0),
+            ZSTDMT_compressionJobParameters {
+                checksumFlag: 0,
+                ldmEnable: ZSTD_PS_DISABLE,
+                nbWorkers: 0,
+            }
+        );
+    }
+
     #[test]
     fn overlap_log_defaults_follow_strategy_groups() {
         assert_eq!(overlap_log_default(ZSTD_FAST), 6);