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/client/client_cli.c

@@ -416,6 +416,7 @@ static const OptionEntry OPTION_TABLE[] = {
 
     {"--max-size", NULL, OPT_ULL, offsetof(Config, max_size)},
     {"--min-size", NULL, OPT_ULL, offsetof(Config, min_size)},
+    {"--one-file-system", "-x", OPT_FLAG, offsetof(Config, one_file_system)},
 };
 
 /* Only boolean options with no required argument are safe to negate. */