feat(cli): move dictionary stat leaf to Rust

Move dictionary-path statting behind the existing Rust CLI file-I/O backend while keeping FIO_getDictFileStat as the C policy adapter. The adapter still owns the NULL-path early return, errno-based EXM_THROW(31) diagnostic, EXM_THROW(32) regular-file policy, and all dictionary orchestration.

FIO_rust_getDictFileStat crosses only the existing path and stat_t boundary, delegates metadata and regular-file classification to UTIL_stat and UTIL_isRegularFileStat, and returns 0/1/2 for success, stat failure, or non-regular input. Reusing those ABIs preserves the C stat_t representation, errno behavior, and stat's symlink-following semantics. Focused backend tests cover regular, missing, directory, NULL, and symlink paths.

Test Plan:
- cargo test --manifest-path rust/cli/Cargo.toml --no-default-features --features cli,compression,decompression --lib fileio_backend (15 passed)
- cargo clippy --no-default-features --features compression, including --benches and --tests (passed)
- CLI cargo clippy with cli,compression,decompression,benchmark, including --benches and --tests (passed)
- cargo +nightly fmt checks for the owned Rust module and CLI crate (passed); root-wide check remains limited by unrelated dict_builder_zdict.rs edits
- Existing CLI dictionary scenarios for missing and directory paths retained status 31/32 and exact diagnostics
- git diff --check and git diff --cached --check (passed)
- zstd build variants were attempted; full links remain blocked by unrelated active dict-builder/zstd-cli/archive edits after the owned C file compiled

programs/fileio.c

@@ -350,6 +350,7 @@ const char* FIO_rust_determineDstName(const char* srcFileName, const char* outDi
 int FIO_rust_adjustMemLimitForPatchFromMode(FIO_prefs_t* prefs,
                                              unsigned long long dictSize,
                                              unsigned long long maxSrcFileSize);
+int FIO_rust_getDictFileStat(const char* fileName, stat_t* statBuf);
 int FIO_rust_setDictBufferMalloc(const char* fileName,
                                  unsigned long long expectedFileSize,
                                  size_t maxSize,
@@ -543,16 +544,26 @@ FIO_openDstFile(FIO_ctx_t* fCtx, FIO_prefs_t* const prefs,
 /* FIO_getDictFileStat() :
  */
 static void FIO_getDictFileStat(const char* fileName, stat_t* dictFileStat) {
+    enum {
+        FIO_DICT_STAT_SUCCESS = 0,
+        FIO_DICT_STAT_FAILED = 1,
+        FIO_DICT_STAT_NON_REGULAR = 2,
+    };
+    int status;
+
     assert(dictFileStat != NULL);
     if (fileName == NULL) return;
 
-    if (!UTIL_stat(fileName, dictFileStat)) {
+    status = FIO_rust_getDictFileStat(fileName, dictFileStat);
+    if (status == FIO_DICT_STAT_FAILED) {
         EXM_THROW(31, "Stat failed on dictionary file %s: %s", fileName, strerror(errno));
     }
 
-    if (!UTIL_isRegularFileStat(dictFileStat)) {
+    if (status == FIO_DICT_STAT_NON_REGULAR) {
         EXM_THROW(32, "Dictionary %s must be a regular file.", fileName);
     }
+
+    assert(status == FIO_DICT_STAT_SUCCESS);
 }
 
 /*  FIO_setDictBufferMalloc() :