refactor(cli): move source-open policy to Rust

Move stdin-sentinel classification and source stat/open result policy into the Rust fileio backend while keeping C responsible for diagnostics, binary-mode setup, and FILE ownership. The bridge leaves stat layout and native file utilities behind the existing C ABI and adds a focused no-stat stdin test.

Test Plan: git diff --cached --check; focused Rust test added but full capped verification will run after the remaining workers are integrated.

programs/fileio.c

@@ -351,6 +351,7 @@ enum {
     FIO_RUST_OPEN_SRC_STAT_FAILED = 1,
     FIO_RUST_OPEN_SRC_NON_REGULAR = 2,
     FIO_RUST_OPEN_SRC_FOPEN_FAILED = 3,
+    FIO_RUST_OPEN_SRC_STDIN = 4,
 };
 int FIO_rust_openSrcFile(int allowBlockDevices,
                          const char* srcFileName,
@@ -671,17 +672,17 @@ static FILE* FIO_openSrcFile(const FIO_prefs_t* const prefs, const char* srcFile
     int allowBlockDevices = prefs != NULL ? prefs->allowBlockDevices : 0;
     assert(srcFileName != NULL);
     assert(statbuf != NULL);
-    if (!strcmp (srcFileName, stdinmark)) {
-        DISPLAYLEVEL(4,"Using stdin for input \n");
-        SET_BINARY_MODE(stdin);
-        return stdin;
-    }
-
     {   FILE* f = NULL;
         int const status = FIO_rust_openSrcFile(allowBlockDevices,
                                                 srcFileName,
                                                 statbuf,
                                                 &f);
+        if (status == FIO_RUST_OPEN_SRC_STDIN) {
+            DISPLAYLEVEL(4,"Using stdin for input \n");
+            SET_BINARY_MODE(stdin);
+            return stdin;
+        }
+
         if (status == FIO_RUST_OPEN_SRC_STAT_FAILED) {
             DISPLAYLEVEL(1, "zstd: can't stat %s : %s -- ignored \n",
                             srcFileName, strerror(errno));