refactor(cli): move highbit policy leaf to Rust

Export FIO_highbit64 directly from Rust under the existing caller symbol and
remove the C forwarding wrapper. Preserve the nonzero-input contract and add
boundary tests for the scalar policy leaf.

Test Plan:
- capped nightly rustfmt check
- capped root clippy with all targets and -D warnings
- capped native make -j1
- capped upstream make -j1 -C tests test
- capped CLI clippy and 185 CLI tests

All integrated checks ran at the combined working-tree tip under a serial
40 GiB virtual-memory cap. Standalone root Rust unit linking remains
unavailable because the crate imports C-owned bridge symbols.

programs/fileio.c

@@ -332,7 +332,8 @@ static int FIO_shouldDisplayMultipleFileSummary(FIO_ctx_t const* fCtx)
  * The declarations in fileio.h remain the C ABI shims while file-I/O
  * orchestration and diagnostics stay here. */
 int FIO_rust_checkFilenameCollisions(const char** filenameTable, unsigned nbFiles);
-unsigned FIO_rust_highbit64(unsigned long long v);
+/* FIO_highbit64() is a Rust-owned scalar policy leaf. */
+unsigned FIO_highbit64(unsigned long long v);
 unsigned long long FIO_rust_getLargestFileSize(const char** inFileNames, unsigned nbFiles);
 int FIO_rust_adjustParamsForPatchFromMode(FIO_prefs_t* prefs,
                                            ZSTD_compressionParameters* comprParams,
@@ -978,16 +979,6 @@ int FIO_checkFilenameCollisions(const char** filenameTable, unsigned nbFiles) {
     return FIO_rust_checkFilenameCollisions(filenameTable, nbFiles);
 }
 
-/* FIO_highbit64() :
- * gives position of highest bit.
- * note : only works for v > 0 !
- */
-static unsigned FIO_highbit64(unsigned long long v)
-{
-    assert(v != 0);
-    return FIO_rust_highbit64(v);
-}
-
 static void FIO_adjustMemLimitForPatchFromMode(FIO_prefs_t* const prefs,
                                     unsigned long long const dictSize,
                                     unsigned long long const maxSrcFileSize)

rust/src/fileio_prefs.rs

@@ -1680,7 +1680,7 @@ fn highbit64(value: u64) -> c_uint {
 }
 
 #[no_mangle]
-pub extern "C" fn FIO_rust_highbit64(value: u64) -> c_uint {
+pub extern "C" fn FIO_highbit64(value: u64) -> c_uint {
     highbit64(value)
 }
 
@@ -3001,8 +3001,9 @@ mod tests {
     #[test]
     fn largest_file_size_scan_preserves_large_values_and_errors() {
         assert_eq!(largest_file_size([0, 1u64 << 63, 7]), 1u64 << 63);
-        assert_eq!(FIO_rust_highbit64(1u64 << 63), 63);
-        assert_eq!(FIO_rust_highbit64(u64::MAX), 63);
+        assert_eq!(FIO_highbit64(1), 0);
+        assert_eq!(FIO_highbit64(1u64 << 63), 63);
+        assert_eq!(FIO_highbit64(u64::MAX), 63);
 
         let missing_path = temporary_file_path("missing");
         let missing = CString::new(missing_path.to_string_lossy().as_bytes()).unwrap();