refactor(cdict): move static initializer error policy to Rust

Keep the private static-CDict initializer in C, but return its native size_t status through the callback boundary. Rust now owns the public success-or-NULL mapping, preserving the original workspace validation, callback order, and private layout ownership while making initializer failures explicit and unit-testable.

Test Plan: ulimit -v 41943040; CARGO_BUILD_JOBS=1 cargo clippy --manifest-path rust/Cargo.toml --all-targets -- -D warnings; cargo +nightly fmt --manifest-path rust/Cargo.toml --all -- --check; GCC and Clang syntax-only checks; make -j1 -C tests test (all 41 shell tests, fuzzer, zstd tester, and zstream tester passed).

lib/compress/zstd_compress.c

@@ -463,7 +463,7 @@ typedef void (*ZSTD_rust_initStaticCDictMoveWorkspace_f)(
         void* context, void* cdict);
 typedef void (*ZSTD_rust_initStaticCDictInitialize_f)(
         void* context, void* cdict, int useRowMatchFinder);
-typedef void* (*ZSTD_rust_initStaticCDictInit_f)(
+typedef size_t (*ZSTD_rust_initStaticCDictInit_f)(
         void* context, void* cdict);
 typedef struct {
     void* callbackContext;
@@ -7306,16 +7306,14 @@ static void ZSTD_rust_initStaticCDict_initialize(
     dictionary->compressionLevel = ZSTD_NO_CLEVEL;
 }
 
-static void* ZSTD_rust_initStaticCDict_init(void* opaque, void* cdict)
+static size_t ZSTD_rust_initStaticCDict_init(void* opaque, void* cdict)
 {
     ZSTD_rust_initStaticCDictContext const* const context =
             (const ZSTD_rust_initStaticCDictContext*)opaque;
-    if (ZSTD_isError( ZSTD_initCDict_internal(
+    return ZSTD_initCDict_internal(
             (ZSTD_CDict*)cdict, context->dict, context->dictSize,
             context->dictLoadMethod, context->dictContentType,
-            context->params) ))
-        return NULL;
-    return cdict;
+            context->params);
 }
 
 const ZSTD_CDict* ZSTD_initStaticCDict(