feat(fileio): move output path construction to Rust

Move the basename-preserving output-directory helper behind the existing Rust utility ABI. Keep the libc allocation contract and platform separator behavior intact while leaving the C file-I/O engines and higher-level naming policy unchanged.

Test Plan:

- cargo test --manifest-path rust/cli/Cargo.toml (93 passed)

- cargo clippy --manifest-path rust/cli/Cargo.toml --all-targets

- make -B -C tests -j2 test-cli-tests (41 passed)

programs/fileio.c

@@ -709,13 +709,8 @@ int FIO_checkFilenameCollisions(const char** filenameTable, unsigned nbFiles) {
     return 0;
 }
 
-static const char*
-extractFilename(const char* path, char separator)
-{
-    const char* search = strrchr(path, separator);
-    if (search == NULL) return path;
-    return search+1;
-}
+char* UTIL_createFilenameFromOutDir(const char* path, const char* outDirName,
+                                    size_t suffixLen);
 
 /* FIO_createFilename_fromOutDir() :
  * Takes a source file name and specified output directory, and
@@ -725,35 +720,7 @@ extractFilename(const char* path, char separator)
 static char*
 FIO_createFilename_fromOutDir(const char* path, const char* outDirName, const size_t suffixLen)
 {
-    const char* filenameStart;
-    char separator;
-    char* result;
-
-#if defined(_MSC_VER) || defined(__MINGW32__) || defined (__MSVCRT__) /* windows support */
-    separator = '\\';
-#else
-    separator = '/';
-#endif
-
-    filenameStart = extractFilename(path, separator);
-#if defined(_MSC_VER) || defined(__MINGW32__) || defined (__MSVCRT__) /* windows support */
-    filenameStart = extractFilename(filenameStart, '/');  /* sometimes, '/' separator is also used on Windows (mingw+msys2) */
-#endif
-
-    result = (char*) calloc(1, strlen(outDirName) + 1 + strlen(filenameStart) + suffixLen + 1);
-    if (!result) {
-        EXM_THROW(30, "zstd: FIO_createFilename_fromOutDir: %s", strerror(errno));
-    }
-
-    memcpy(result, outDirName, strlen(outDirName));
-    if (outDirName[strlen(outDirName)-1] == separator) {
-        memcpy(result + strlen(outDirName), filenameStart, strlen(filenameStart));
-    } else {
-        memcpy(result + strlen(outDirName), &separator, 1);
-        memcpy(result + strlen(outDirName) + 1, filenameStart, strlen(filenameStart));
-    }
-
-    return result;
+    return UTIL_createFilenameFromOutDir(path, outDirName, suffixLen);
 }
 
 /* FIO_highbit64() :