feat(cli): move multi-file output policy to Rust

Port the scalar decision matrix for multi-input concatenation behind the CLI Rust ABI. C retains warnings, preference mutation, fatal diagnostics, and confirmation prompts; Rust now classifies protection, remove-file disabling, quiet abort, and confirmation paths with focused coverage.

Test Plan:

- cargo test --manifest-path rust/cli/Cargo.toml fileio_prefs::tests::multi_files_concat

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

- cargo clippy --manifest-path rust/cli/Cargo.toml --benches

- cargo clippy --manifest-path rust/cli/Cargo.toml --tests

- cargo +nightly fmt --manifest-path rust/Cargo.toml --all

- make -B -C programs -j2 zstd

- make -C tests -j2 test-cli-tests

rust/src/fileio_prefs.rs

@@ -22,6 +22,12 @@ const FIO_LDM_PARAM_NOTSET: c_int = 9999;
 const FIO_PATCH_MEM_LIMIT_SUCCESS: c_int = 0;
 const FIO_PATCH_MEM_LIMIT_UNKNOWN_SIZE: c_int = 1;
 const FIO_PATCH_MEM_LIMIT_TOO_LARGE: c_int = 2;
+const FIO_MULTI_FILES_ACTION_PROCEED: c_int = 0;
+const FIO_MULTI_FILES_ACTION_FATAL_STDOUT_REMOVE: c_int = 1;
+const FIO_MULTI_FILES_ACTION_FATAL_TEST_REMOVE: c_int = 2;
+const FIO_MULTI_FILES_ACTION_DISABLE_REMOVE: c_int = 3;
+const FIO_MULTI_FILES_ACTION_QUIET_ABORT: c_int = 4;
+const FIO_MULTI_FILES_ACTION_CONFIRM: c_int = 5;
 const UTIL_FILESIZE_UNKNOWN: u64 = u64::MAX;
 const ZSTD_WINDOWLOG_MAX: u32 = if size_of::<usize>() == 4 { 30 } else { 31 };
 const ZSTD_BTLAZY2: c_int = 6;
@@ -603,6 +609,67 @@ pub extern "C" fn FIO_rust_cycleLog(hash_log: c_uint, strategy: c_int) -> c_uint
     cycle_log(hash_log, strategy)
 }
 
+#[allow(clippy::too_many_arguments)]
+#[inline]
+fn multi_files_concat_action(
+    nb_files_total: c_int,
+    has_stdout_output: c_int,
+    test_mode: c_int,
+    has_output_file: c_int,
+    remove_src_file: c_int,
+    overwrite: c_int,
+    display_level: c_int,
+    display_level_cutoff: c_int,
+) -> c_int {
+    if has_stdout_output != 0 && remove_src_file != 0 {
+        return FIO_MULTI_FILES_ACTION_FATAL_STDOUT_REMOVE;
+    }
+    if test_mode != 0 && remove_src_file != 0 {
+        return FIO_MULTI_FILES_ACTION_FATAL_TEST_REMOVE;
+    }
+    if test_mode != 0 || nb_files_total <= 1 || has_output_file == 0 {
+        return FIO_MULTI_FILES_ACTION_PROCEED;
+    }
+    if remove_src_file != 0 {
+        return FIO_MULTI_FILES_ACTION_DISABLE_REMOVE;
+    }
+    if has_stdout_output != 0 || overwrite != 0 {
+        return FIO_MULTI_FILES_ACTION_PROCEED;
+    }
+    if display_level <= display_level_cutoff {
+        FIO_MULTI_FILES_ACTION_QUIET_ABORT
+    } else {
+        FIO_MULTI_FILES_ACTION_CONFIRM
+    }
+}
+
+/// Select the warning/prompt action for multiple inputs sharing one output.
+///
+/// C retains the user-facing diagnostics, preference mutation, and prompt;
+/// Rust only evaluates the scalar policy and returns the action enum.
+#[no_mangle]
+pub extern "C" fn FIO_rust_multiFilesConcatAction(
+    nb_files_total: c_int,
+    has_stdout_output: c_int,
+    test_mode: c_int,
+    has_output_file: c_int,
+    remove_src_file: c_int,
+    overwrite: c_int,
+    display_level: c_int,
+    display_level_cutoff: c_int,
+) -> c_int {
+    multi_files_concat_action(
+        nb_files_total,
+        has_stdout_output,
+        test_mode,
+        has_output_file,
+        remove_src_file,
+        overwrite,
+        display_level,
+        display_level_cutoff,
+    )
+}
+
 fn largest_file_size<I>(sizes: I) -> u64
 where
     I: IntoIterator<Item = u64>,
@@ -1112,6 +1179,72 @@ mod tests {
         assert_eq!(FIO_rust_cycleLog(20, ZSTD_BTLAZY2 + 3), 19);
     }
 
+    #[test]
+    fn multi_files_concat_policy_protects_stdout_and_test_mode_removal() {
+        assert_eq!(
+            multi_files_concat_action(3, 1, 0, 1, 1, 0, 2, 1),
+            FIO_MULTI_FILES_ACTION_FATAL_STDOUT_REMOVE
+        );
+        assert_eq!(
+            multi_files_concat_action(3, 0, 1, 1, 1, 0, 2, 1),
+            FIO_MULTI_FILES_ACTION_FATAL_TEST_REMOVE
+        );
+        assert_eq!(
+            multi_files_concat_action(3, 0, 1, 1, 0, 0, 2, 1),
+            FIO_MULTI_FILES_ACTION_PROCEED
+        );
+    }
+
+    #[test]
+    fn multi_files_concat_policy_keeps_nonconcatenating_paths_quiet() {
+        for (nb_files, has_output_file) in [(1, 1), (3, 0)] {
+            assert_eq!(
+                multi_files_concat_action(nb_files, 0, 0, has_output_file, 0, 0, 2, 1),
+                FIO_MULTI_FILES_ACTION_PROCEED
+            );
+        }
+    }
+
+    #[test]
+    fn multi_files_concat_policy_disables_remove_before_other_decisions() {
+        assert_eq!(
+            multi_files_concat_action(3, 0, 0, 1, 1, 0, 2, 1),
+            FIO_MULTI_FILES_ACTION_DISABLE_REMOVE
+        );
+        assert_eq!(
+            multi_files_concat_action(3, 0, 0, 1, 0, 0, 1, 1),
+            FIO_MULTI_FILES_ACTION_QUIET_ABORT
+        );
+    }
+
+    #[test]
+    fn multi_files_concat_policy_allows_stdout_and_overwrite() {
+        assert_eq!(
+            multi_files_concat_action(3, 1, 0, 1, 0, 0, 0, 1),
+            FIO_MULTI_FILES_ACTION_PROCEED
+        );
+        assert_eq!(
+            multi_files_concat_action(3, 0, 0, 1, 0, 1, 0, 1),
+            FIO_MULTI_FILES_ACTION_PROCEED
+        );
+    }
+
+    #[test]
+    fn multi_files_concat_policy_uses_quiet_and_confirmation_boundaries() {
+        assert_eq!(
+            multi_files_concat_action(3, 0, 0, 1, 0, 0, 1, 1),
+            FIO_MULTI_FILES_ACTION_QUIET_ABORT
+        );
+        assert_eq!(
+            multi_files_concat_action(3, 0, 0, 1, 0, 0, 2, 1),
+            FIO_MULTI_FILES_ACTION_CONFIRM
+        );
+        assert_eq!(
+            FIO_rust_multiFilesConcatAction(3, 0, 0, 1, 0, 0, 2, 1),
+            FIO_MULTI_FILES_ACTION_CONFIRM
+        );
+    }
+
     #[test]
     fn buffer_shims_preserve_fields_and_c_layout() {
         let input_word = size_of::<*const c_void>();