feat(fileio): move size helpers to Rust

Move the file-stat size scan and highest-bit helper behind Rust ABI implementations while preserving the C file-I/O surface and unknown-size sentinel behavior. Add temporary-file and boundary coverage for the migrated helpers.

Test Plan: cargo test --manifest-path rust/cli/Cargo.toml --no-default-features --features cli,compression,decompression,benchmark; cargo test --manifest-path rust/cli/Cargo.toml --no-default-features --features helpers; cargo clippy --manifest-path rust/cli/Cargo.toml --all-targets --no-default-features --features cli,compression,decompression,benchmark; cargo clippy --manifest-path rust/cli/Cargo.toml --all-targets --no-default-features --features helpers; make -B -C programs -j2 zstd zstd-small zstd-frugal; make -B -C tests -j2 test-cli-tests

rust/src/fileio_prefs.rs

@@ -476,6 +476,47 @@ pub unsafe extern "C" fn FIO_determineHasStdinInput(
     }
 }
 
+/// Return the position of the highest set bit in a non-zero 64-bit value.
+///
+/// This mirrors `FIO_highbit64()` and deliberately keeps its zero-input
+/// assertion semantics.  In builds where assertions are disabled, zero
+/// still produces the same result as the original shift loop.
+#[no_mangle]
+pub extern "C" fn FIO_rust_highbit64(value: u64) -> c_uint {
+    debug_assert!(value != 0);
+    if value == 0 {
+        return 0;
+    }
+    (u64::BITS - 1 - value.leading_zeros()) as c_uint
+}
+
+fn largest_file_size<I>(sizes: I) -> u64
+where
+    I: IntoIterator<Item = u64>,
+{
+    let mut largest = 0;
+    for size in sizes {
+        largest = size.max(largest);
+    }
+    largest
+}
+
+/// Rust implementation of the size scan used by `FIO_getLargestFileSize`.
+///
+/// `UTIL_getFileSize()` remains responsible for the actual stat operation.
+/// In particular, an unknown size is `UTIL_FILESIZE_UNKNOWN` and therefore
+/// wins the max scan exactly as it does in the original C implementation.
+#[no_mangle]
+pub unsafe extern "C" fn FIO_rust_getLargestFileSize(
+    file_names: *const *const c_char,
+    nb_files: c_uint,
+) -> u64 {
+    largest_file_size(
+        (0..nb_files as usize)
+            .map(|index| unsafe { crate::util::UTIL_getFileSize(*file_names.add(index)) }),
+    )
+}
+
 #[inline]
 fn filename_path_separator() -> u8 {
     if cfg!(windows) {
@@ -561,8 +602,54 @@ pub unsafe extern "C" fn FIO_rust_checkFilenameCollisions(
 mod tests {
     use super::*;
     use std::ffi::CString;
+    use std::fs;
     use std::mem::{align_of, offset_of, size_of};
 
+    const UTIL_FILESIZE_UNKNOWN: u64 = u64::MAX;
+
+    fn temporary_file_path(name: &str) -> std::path::PathBuf {
+        std::env::temp_dir().join(format!("zstd-fileio-prefs-{}-{name}", std::process::id()))
+    }
+
+    #[test]
+    fn largest_file_size_scan_handles_empty_input() {
+        assert_eq!(unsafe { FIO_rust_getLargestFileSize(ptr::null(), 0) }, 0);
+    }
+
+    #[test]
+    fn largest_file_size_scan_handles_ordinary_files() {
+        let smaller_path = temporary_file_path("smaller");
+        let larger_path = temporary_file_path("larger");
+        fs::write(&smaller_path, b"small").unwrap();
+        fs::write(&larger_path, b"larger file").unwrap();
+        let smaller = CString::new(smaller_path.to_string_lossy().as_bytes()).unwrap();
+        let larger = CString::new(larger_path.to_string_lossy().as_bytes()).unwrap();
+        let names = [smaller.as_ptr(), larger.as_ptr()];
+
+        assert_eq!(
+            unsafe { FIO_rust_getLargestFileSize(names.as_ptr(), names.len() as c_uint) },
+            11
+        );
+
+        fs::remove_file(smaller_path).unwrap();
+        fs::remove_file(larger_path).unwrap();
+    }
+
+    #[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);
+
+        let missing_path = temporary_file_path("missing");
+        let missing = CString::new(missing_path.to_string_lossy().as_bytes()).unwrap();
+        let names = [missing.as_ptr()];
+        assert_eq!(
+            unsafe { FIO_rust_getLargestFileSize(names.as_ptr(), 1) },
+            UTIL_FILESIZE_UNKNOWN
+        );
+    }
+
     #[test]
     fn c_layouts_match_the_headers() {
         let word = size_of::<usize>();