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/client/scanner.h

@@ -4,14 +4,16 @@
 #include "chunk.h"
 #include "queue.h"
 #include <dirent.h>
+#include <stdbool.h>
 
 typedef struct {
   Queue *directories;
   DIR *current_dir;
   char *current_path;
+  bool use_metadata;
 } DirectoryScanner;
 
-DirectoryScanner *directory_scanner_create(char *root_directory);
+DirectoryScanner *directory_scanner_create(char *root_directory, bool use_metadata);
 Chunk *directory_scanner_next(DirectoryScanner *scanner);
 void directory_scanner_destroy(DirectoryScanner *scanner);