refactor(compress): export context parameter policy from Rust

Move ZSTD_getCParamsFromCCtxParams() out of its C forwarding body and
export the original symbol from the Rust parameter API. The Rust-side
ABI mirror already has layout checks, so it can project the same fields
to the existing scalar policy leaf without exposing additional context
state. Keep the exclusion mask in C and query it from Rust so reduced
builds retain their preprocessor-selected block-compressor behavior.

Test Plan:
- `cc -fsyntax-only -Ilib -Ilib/common -Ilib/compress -Ilib/decompress -Ilib/dict -Ilib/legacy lib/compress/zstd_compress.c` -- passed.
- `cargo +nightly fmt --manifest-path rust/Cargo.toml --all -- --check` -- passed.
- `git diff --check` and `git diff --cached --check` -- passed.
- Heavy Cargo, Make, native, and test-suite verification intentionally not run per request.

lib/compress/zstd_compress.c

@@ -3924,17 +3924,7 @@ U32 ZSTD_getCParamsExclusionMask(void)
     return mask;
 }
 
-ZSTD_compressionParameters ZSTD_getCParamsFromCCtxParams(
+/* ZSTD_getCParamsFromCCtxParams() is implemented directly by Rust. */
-        const ZSTD_CCtx_params* CCtxParams, U64 srcSizeHint, size_t dictSize, ZSTD_CParamMode_e mode)
-{
-    return ZSTD_rust_params_getCParamsFromCCtxParams(
-            CCtxParams->compressionLevel, CCtxParams->srcSizeHint,
-            srcSizeHint, dictSize, (int)mode,
-            (int)CCtxParams->ldmParams.enableLdm,
-            ZSTD_LDM_DEFAULT_WINDOW_LOG, CCtxParams->cParams,
-            (int)CCtxParams->useRowMatchFinder,
-            ZSTD_getCParamsExclusionMask());
-}
 
 static size_t ZSTD_estimateCCtxSize_usingCCtxParams_internal(
         const ZSTD_compressionParameters* cParams,

rust/src/zstd_compress_params_api.rs

@@ -129,6 +129,7 @@ pub struct ZSTD_CCtx_params {
 
 const DEFAULT_CLEVEL: c_int = 3;
 const NO_CLEVEL: c_int = 0;
+const ZSTD_LDM_DEFAULT_WINDOW_LOG: u32 = 27;
 const PS_AUTO: c_int = 0;
 const PS_DISABLE: c_int = 2;
 
@@ -204,12 +205,23 @@ pub extern "C" fn ZSTD_rust_isUpdateAuthorized(param: c_int) -> c_int {
 
 #[cfg(not(test))]
 unsafe extern "C" {
+    fn ZSTD_getCParamsExclusionMask() -> u32;
     fn ZSTD_rust_cctx_params_is_multithreaded() -> c_int;
     fn ZSTD_rust_cctx_params_nb_workers_max() -> c_int;
     fn ZSTD_rust_cctx_params_job_size_min() -> c_int;
     fn ZSTD_rust_cctx_params_job_size_max() -> c_int;
 }
 
+#[cfg(not(test))]
+fn cparams_exclusion_mask() -> u32 {
+    unsafe { ZSTD_getCParamsExclusionMask() }
+}
+
+#[cfg(test)]
+fn cparams_exclusion_mask() -> u32 {
+    0
+}
+
 #[derive(Clone, Copy)]
 struct BoundsConfig {
     multithreaded: bool,
@@ -582,6 +594,34 @@ pub unsafe extern "C" fn ZSTD_rust_CCtxParams_setZstdParams(
     unsafe { set_zstd_params_impl(cctx_params, zstd_params) }
 }
 
+/// Rust implementation of the private `ZSTD_getCParamsFromCCtxParams()`
+/// policy leaf.
+///
+/// The `ZSTD_CCtx_params` mirror is layout-checked in this module. The
+/// exclusion mask remains supplied by C so build-time block-compressor
+/// configuration continues to follow the C preprocessor configuration.
+#[no_mangle]
+pub unsafe extern "C" fn ZSTD_getCParamsFromCCtxParams(
+    cctx_params: *const ZSTD_CCtx_params,
+    src_size_hint: u64,
+    dict_size: usize,
+    mode: c_int,
+) -> ZSTD_compressionParameters {
+    let cctx_params = unsafe { &*cctx_params };
+    ZSTD_rust_params_getCParamsFromCCtxParams(
+        cctx_params.compressionLevel,
+        cctx_params.srcSizeHint,
+        src_size_hint,
+        dict_size,
+        mode,
+        cctx_params.ldmParams.enableLdm,
+        ZSTD_LDM_DEFAULT_WINDOW_LOG,
+        cctx_params.cParams,
+        cctx_params.useRowMatchFinder,
+        cparams_exclusion_mask(),
+    )
+}
+
 /// Prepares the parameter subset used by `ZSTD_createCDict_advanced2()`.
 ///
 /// The C dictionary allocator remains responsible for allocation and
@@ -1334,6 +1374,39 @@ mod tests {
         );
     }
 
+    #[test]
+    fn direct_cparams_projection_matches_the_scalar_policy_leaf() {
+        let mut storage = MaybeUninit::<ZSTD_CCtx_params>::zeroed();
+        let params = storage.as_mut_ptr();
+
+        unsafe {
+            assert_eq!(ZSTD_CCtxParams_init(params, DEFAULT_CLEVEL), 0);
+            (*params).srcSizeHint = 16 * 1024;
+
+            let expected = ZSTD_rust_params_getCParamsFromCCtxParams(
+                (*params).compressionLevel,
+                (*params).srcSizeHint,
+                ZSTD_CONTENTSIZE_UNKNOWN,
+                0,
+                ZSTD_RUST_CPM_NO_ATTACH_DICT,
+                (*params).ldmParams.enableLdm,
+                ZSTD_LDM_DEFAULT_WINDOW_LOG,
+                (*params).cParams,
+                (*params).useRowMatchFinder,
+                0,
+            );
+            assert_eq!(
+                ZSTD_getCParamsFromCCtxParams(
+                    params,
+                    ZSTD_CONTENTSIZE_UNKNOWN,
+                    0,
+                    ZSTD_RUST_CPM_NO_ATTACH_DICT,
+                ),
+                expected
+            );
+        }
+    }
+
     #[test]
     fn target_c_block_size_predicate_uses_zero_as_the_disable_value() {
         let mut storage = MaybeUninit::<ZSTD_CCtx_params>::zeroed();