refactor: remove dead API, rename to_disk/config_is_remote_dest, fix perror newlines

- Delete unused public array_list_extend (made static)
- Delete legacy 16-parameter parallel_scanner_create wrapper; migrate test to parallel_scanner_create_with_options
- Rename to_disk -> file_write_to_disk and is_remote_dest -> config_is_remote_dest for module_action naming convention
- Remove stray newlines in perror calls (perror already appends one)

tests/test_metadata.c

@@ -125,7 +125,7 @@ static void test_metadata_rejects_invalid_values() {
 static void test_file_restore_metadata() {
   const char* path = "temp_meta_restore_test.txt";
   const char* content = "test content";
-  EXPECT_TRUE(to_disk(path, content, strlen(content), false, false));
+  EXPECT_TRUE(file_write_to_disk(path, content, strlen(content), false, false));
 
   FileMetadata m;
   m.mode = 0644;