refactor: merge File/FileReceive, add optional metadata transfer with -M flag

- Remove FileReceive struct; use File everywhere with nullable FileMetadata*
- Remove struct stat from File; file size lives in Data->size (data_create_reserve)
- Add FileMetadata struct (mode, uid, gid, mtime) sent conditionally over wire
- Add -M / --preserve flag to client
- Restore permissions, ownership, timestamps on disk write
- Wire format uses per-file present flag for metadata (zero overhead when off)

src/shared/file.h

@@ -5,24 +5,27 @@
 #include <sys/stat.h>
 
 typedef struct {
-  char *path;
-  struct stat stats;
-  Data *data;
-} File;
+  mode_t mode;
+  uid_t uid;
+  gid_t gid;
+  time_t mtime_sec;
+  long mtime_nsec;
+} FileMetadata;
 
 typedef struct {
   char *path;
   Data *data;
-} FileReceive;
+  FileMetadata *metadata;
+} File;
 
-File *file_create(const char *path, struct stat *stats);
+File *file_create(const char *path);
 void file_destroy(void *item);
 void file_load_data(File *file);
 void file_print(void *item);
 void file_send_single_calls(File *file, int file_descriptor);
 size_t file_content_to_buffer(File *file);
-
-FileReceive *file_receive_create(char *path, Data *data);
-void file_receive_destroy(void *file_receive);
+FileMetadata *file_metadata_create(struct stat *stats);
+void file_metadata_destroy(void *metadata);
+FileMetadata *file_receive_metadata(int file_descriptor);
 
 #endif