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.

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();