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.

RSYNC_COMPAT.md

@@ -64,7 +64,7 @@ This document maps rsync's full feature set to FastSync's current implementation
 | `--existing` | Skip creating new files on receiver | ✅ Implemented | Existing destination files continue through normal update handling |
 | `--ignore-existing` | Skip updating existing files | ❌ Not Implemented | |
 | `--remove-source-files` | Sender removes regular files after confirmed transfer | ✅ Implemented | |
-| `-x`, `--one-file-system` | Do not cross filesystem boundaries | ❌ Not Implemented | |
+| `-x`, `--one-file-system` | Do not cross filesystem boundaries | ✅ Implemented | Sender scanner captures the root device and skips descending into mount-point crossings (`st_dev` differs) |
 | `-F` | Add the default `.rsync-filter` rules | ❌ Not Implemented | |
 
 ## 4. Directory Options