feat: add -x/--one-file-system to stay within the source filesystem

Capture the transfer root's device (st_dev) at scanner creation and skip
descending into any subdirectory on a different device (a mount point).
Implemented sender/client-side only: sequential BFS and parallel (-m) root
scan apply the same scanner_same_filesystem decision; no wire/protocol change
and default behavior is unchanged. Unit tests cover the pure decision, same
device scanning in both modes, and CLI parsing; integration tests prove -x
leaves a single-filesystem tree byte-identical and, when root can mount a
tmpfs, skips a genuine cross-device subtree.

src/shared/config.h

@@ -104,6 +104,7 @@ typedef struct Config {
   char* files_from;
   bool cvs_exclude;
   bool prune_empty_dirs;
+  bool one_file_system; /* -x/--one-file-system: do not cross filesystem boundaries */
   bool relative;
 
   // Issue #130: Remote shell/connection options