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/data.h

@@ -9,6 +9,7 @@ typedef struct {
 } Data;
 
 Data *data_create_empty(size_t data_size);
+Data *data_create_reserve(size_t size);
 Data *data_create(void *data, size_t data_size);
 void data_destroy(Data *data);
 Data *data_compress(Data *data_to_compress, int compression_level);