feat(compress): move CParamMode policy to Rust

Keep C responsible for extracting the cdict and CCtx scalar fields while
moving the attach-versus-copy mode decision into the Rust parameter module.
The Rust ABI short-circuits absent cdicts and reuses the established
attachment predicate, preserving unknown-size, force-attach, force-copy, and
force-window behavior.

Test Plan:
- cargo test --manifest-path rust/Cargo.toml --no-default-features
  --features compression
  cparam_mode_preserves_cdict_presence_and_attachment_policy -- passed
- make -B -C lib -j2 lib -- passed
- make -C tests -j2 fuzzer -- passed
- ./tests/fuzzer -s4560 -t47 -i48 -v -- passed
- Required clippy normal/benches/tests, nightly fmt, and diff checks -- passed

lib/compress/zstd_compress.c

@@ -134,6 +134,11 @@ U32 ZSTD_rust_params_dedicatedDictSearch_revertHashLog(U32 hashLog);
 int ZSTD_rust_params_shouldAttachDict(int strategy, int dedicatedDictSearch,
                                       U64 pledgedSrcSize, int attachDictPref,
                                       int forceWindow);
+int ZSTD_rust_params_getCParamMode(int cdict_present, int cdict_strategy,
+                                   int cdict_dedicated_search,
+                                   U64 pledgedSrcSize,
+                                   int params_attachDictPref,
+                                   int params_forceWindow);
 
 /* CCtx parameter state is mirrored by rust/src/zstd_compress_params_api.rs.
  * Rust owns parameter bounds and clamping; C exposes only the
@@ -4400,10 +4405,16 @@ size_t ZSTD_CStreamOutSize(void)
 
 static ZSTD_CParamMode_e ZSTD_getCParamMode(ZSTD_CDict const* cdict, ZSTD_CCtx_params const* params, U64 pledgedSrcSize)
 {
-    if (cdict != NULL && ZSTD_shouldAttachDict(cdict, params, pledgedSrcSize))
-        return ZSTD_cpm_attachDict;
-    else
-        return ZSTD_cpm_noAttachDict;
+    int const cdict_present = cdict != NULL;
+    int const cdict_strategy = cdict_present ? cdict->matchState.cParams.strategy : 0;
+    int const cdict_dedicated_search = cdict_present ? cdict->matchState.dedicatedDictSearch : 0;
+    return (ZSTD_CParamMode_e)ZSTD_rust_params_getCParamMode(
+            cdict_present,
+            cdict_strategy,
+            cdict_dedicated_search,
+            pledgedSrcSize,
+            (int)params->attachDictPref,
+            params->forceWindow);
 }
 
 /* ZSTD_resetCStream():

rust/src/zstd_compress_params.rs

@@ -485,6 +485,30 @@ fn should_attach_dict(
             && force_window == 0)
 }
 
+#[inline]
+fn get_cparam_mode(
+    cdict_present: c_int,
+    cdict_strategy: c_int,
+    cdict_dedicated_search: c_int,
+    pledged_src_size: u64,
+    params_attach_dict_pref: c_int,
+    params_force_window: c_int,
+) -> c_int {
+    if cdict_present != 0
+        && should_attach_dict(
+            cdict_strategy,
+            cdict_dedicated_search,
+            pledged_src_size,
+            params_attach_dict_pref,
+            params_force_window,
+        )
+    {
+        ZSTD_RUST_CPM_ATTACH_DICT
+    } else {
+        ZSTD_RUST_CPM_NO_ATTACH_DICT
+    }
+}
+
 /// C ABI for `ZSTD_CDictIndicesAreTagged()`.
 #[no_mangle]
 pub extern "C" fn ZSTD_rust_params_cdictIndicesAreTagged(
@@ -531,6 +555,26 @@ pub extern "C" fn ZSTD_rust_params_shouldAttachDict(
     ))
 }
 
+/// C ABI for `ZSTD_getCParamMode()`.
+#[no_mangle]
+pub extern "C" fn ZSTD_rust_params_getCParamMode(
+    cdict_present: c_int,
+    cdict_strategy: c_int,
+    cdict_dedicated_search: c_int,
+    pledged_src_size: u64,
+    params_attach_dict_pref: c_int,
+    params_force_window: c_int,
+) -> c_int {
+    get_cparam_mode(
+        cdict_present,
+        cdict_strategy,
+        cdict_dedicated_search,
+        pledged_src_size,
+        params_attach_dict_pref,
+        params_force_window,
+    )
+}
+
 #[inline]
 fn dict_and_window_log(window_log: u32, src_size: u64, dict_size: u64) -> u32 {
     /* 1ULL << ZSTD_WINDOWLOG_MAX, which is smaller for 32-bit builds. */
@@ -1468,6 +1512,55 @@ mod tests {
         );
     }
 
+    #[test]
+    fn cparam_mode_preserves_cdict_presence_and_attachment_policy() {
+        assert_eq!(
+            ZSTD_rust_params_getCParamMode(0, ZSTD_FAST, 0, 1, ZSTD_DICT_FORCE_ATTACH, 0,),
+            ZSTD_RUST_CPM_NO_ATTACH_DICT
+        );
+        assert_eq!(
+            ZSTD_rust_params_getCParamMode(1, ZSTD_FAST, 0, 8 * 1024, 0, 0),
+            ZSTD_RUST_CPM_ATTACH_DICT
+        );
+        assert_eq!(
+            ZSTD_rust_params_getCParamMode(
+                1,
+                ZSTD_FAST,
+                0,
+                8 * 1024 + 1,
+                ZSTD_DICT_FORCE_ATTACH,
+                0,
+            ),
+            ZSTD_RUST_CPM_ATTACH_DICT
+        );
+        assert_eq!(
+            ZSTD_rust_params_getCParamMode(
+                1,
+                ZSTD_FAST,
+                0,
+                ZSTD_CONTENTSIZE_UNKNOWN,
+                ZSTD_DICT_FORCE_COPY,
+                0,
+            ),
+            ZSTD_RUST_CPM_NO_ATTACH_DICT
+        );
+        assert_eq!(
+            ZSTD_rust_params_getCParamMode(1, ZSTD_FAST, 0, ZSTD_CONTENTSIZE_UNKNOWN, 0, 0,),
+            ZSTD_RUST_CPM_ATTACH_DICT
+        );
+        assert_eq!(
+            ZSTD_rust_params_getCParamMode(
+                1,
+                ZSTD_FAST,
+                0,
+                ZSTD_CONTENTSIZE_UNKNOWN,
+                ZSTD_DICT_FORCE_ATTACH,
+                1,
+            ),
+            ZSTD_RUST_CPM_NO_ATTACH_DICT
+        );
+    }
+
     #[test]
     fn external_sequence_validation_preserves_its_int_mode() {
         for mode in [c_int::MIN, -1, 0, 1, c_int::MAX] {