refactor(compress): move MT parameter update policy to Rust

Move the next-job parameter derivation in
ZSTDMT_updateCParams_whileCompressing() behind a scalar Rust projection. Rust
now reproduces the existing CCtx-parameter derivation and restores the active
frame's saved windowLog; C retains private MT/CCtx layouts, logging, and final
writes.

Expose the preprocessor-derived block-compressor exclusion mask through the
private C boundary so reduced builds keep their authoritative configuration.
Add C/Rust layout assertions and focused tests for window preservation and
strategy-exclusion routing.

Test Plan:
- `ulimit -v 41943040; cargo +nightly fmt --manifest-path rust/Cargo.toml --all -- --check`
- `ulimit -v 41943040; CARGO_BUILD_JOBS=1 cargo clippy --manifest-path rust/Cargo.toml --all-targets -- -D warnings`
- `ulimit -v 41943040; CARGO_BUILD_JOBS=1 cargo clippy --manifest-path rust/cli/Cargo.toml --all-targets -- -D warnings`
- `ulimit -v 41943040; CARGO_BUILD_JOBS=1 cargo test --manifest-path rust/cli/Cargo.toml --all-targets` (184 tests)
- `ulimit -v 41943040; make -j1`
- `ulimit -v 41943040; make -j1 -C tests test` (all upstream tests completed successfully)
- Standalone Rust unit linking remains unavailable without the C-owned bridge symbols; integrated build and upstream MT/API coverage passed.

lib/compress/zstd_compress.c

@@ -3945,7 +3945,7 @@ U32 ZSTD_cycleLog(U32 hashLog, ZSTD_strategy strat)
     return ZSTD_rust_params_cycleLog(hashLog, (int)strat);
 }
 
-static U32 ZSTD_getCParamsExclusionMask(void);
+U32 ZSTD_getCParamsExclusionMask(void);
 
 /** ZSTD_adjustCParams_internal() :
  *  optimize `cPar` for a specified input (`srcSize` and `dictSize`).
@@ -4001,7 +4001,7 @@ enum {
     ZSTD_RUST_PARAMS_EXCLUDE_DFAST = 1u << 6
 };
 
-static U32 ZSTD_getCParamsExclusionMask(void)
+U32 ZSTD_getCParamsExclusionMask(void)
 {
     U32 mask = 0;
 #ifdef ZSTD_EXCLUDE_BTULTRA_BLOCK_COMPRESSOR

lib/compress/zstdmt_compress.c

@@ -25,6 +25,53 @@
 #include "zstd_ldm.h"
 #include "zstdmt_compress.h"
 
+/* The Rust parameter leaf receives only this scalar snapshot.  Keep the
+ * private ZSTDMT_CCtx and ZSTD_CCtx_params layouts in this translation unit. */
+typedef struct {
+    int compressionLevel;
+    int cctxSrcSizeHint;
+    U64 srcSizeHint;
+    size_t dictSize;
+    int mode;
+    int enableLdm;
+    U32 ldmDefaultWindowLog;
+    ZSTD_compressionParameters overrides;
+    int useRowMatchFinder;
+    U32 exclusionMask;
+    U32 savedWindowLog;
+} ZSTDMT_RustCParamsUpdateProjection;
+typedef char ZSTDMT_rust_cparams_update_projection_layout[
+        (offsetof(ZSTDMT_RustCParamsUpdateProjection, compressionLevel) == 0
+         && offsetof(ZSTDMT_RustCParamsUpdateProjection, cctxSrcSizeHint)
+                == sizeof(int)
+         && offsetof(ZSTDMT_RustCParamsUpdateProjection, srcSizeHint)
+                == 2 * sizeof(int)
+         && offsetof(ZSTDMT_RustCParamsUpdateProjection, dictSize) == 16
+         && offsetof(ZSTDMT_RustCParamsUpdateProjection, mode)
+                == (sizeof(void*) == 8 ? 24 : 20)
+         && offsetof(ZSTDMT_RustCParamsUpdateProjection, enableLdm)
+                == (sizeof(void*) == 8 ? 28 : 24)
+         && offsetof(ZSTDMT_RustCParamsUpdateProjection, ldmDefaultWindowLog)
+                == (sizeof(void*) == 8 ? 32 : 28)
+         && offsetof(ZSTDMT_RustCParamsUpdateProjection, overrides)
+                == (sizeof(void*) == 8 ? 36 : 32)
+         && offsetof(ZSTDMT_RustCParamsUpdateProjection, useRowMatchFinder)
+                == (sizeof(void*) == 8 ? 64 : 60)
+         && offsetof(ZSTDMT_RustCParamsUpdateProjection, exclusionMask)
+                == (sizeof(void*) == 8 ? 68 : 64)
+         && offsetof(ZSTDMT_RustCParamsUpdateProjection, savedWindowLog)
+                == (sizeof(void*) == 8 ? 72 : 68)
+         && sizeof(ZSTDMT_RustCParamsUpdateProjection)
+                == (sizeof(void*) == 8 ? 80 : 72))
+        ? 1 : -1];
+
+ZSTD_compressionParameters ZSTD_rust_params_updateCParamsWhileCompressing(
+        ZSTDMT_RustCParamsUpdateProjection projection);
+
+/* Defined in zstd_compress.c so all C translation units use the same
+ * preprocessor-derived block-compressor exclusion mask. */
+U32 ZSTD_getCParamsExclusionMask(void);
+
 /* Guards code to support resizing the SeqPool.
  * We will want to resize the SeqPool to save memory in the future.
  * Until then, comment the code out since it is unused.
@@ -2260,9 +2307,21 @@ void ZSTDMT_updateCParams_whileCompressing(ZSTDMT_CCtx* mtctx, const ZSTD_CCtx_p
     DEBUGLOG(5, "ZSTDMT_updateCParams_whileCompressing (level:%i)",
                 compressionLevel);
     mtctx->params.compressionLevel = compressionLevel;
-    {   ZSTD_compressionParameters cParams = ZSTD_getCParamsFromCCtxParams(cctxParams, ZSTD_CONTENTSIZE_UNKNOWN, 0, ZSTD_cpm_noAttachDict);
-        cParams.windowLog = saved_wlog;
-        mtctx->params.cParams = cParams;
+    {   ZSTDMT_RustCParamsUpdateProjection const projection = {
+            cctxParams->compressionLevel,
+            cctxParams->srcSizeHint,
+            ZSTD_CONTENTSIZE_UNKNOWN,
+            0,
+            ZSTD_cpm_noAttachDict,
+            cctxParams->ldmParams.enableLdm,
+            ZSTD_LDM_DEFAULT_WINDOW_LOG,
+            cctxParams->cParams,
+            cctxParams->useRowMatchFinder,
+            ZSTD_getCParamsExclusionMask(),
+            saved_wlog
+        };
+        mtctx->params.cParams =
+                ZSTD_rust_params_updateCParamsWhileCompressing(projection);
     }
 }