rsync flags: -a, -n, -p, --exclude, --delete

Feature changes across 15 files:

- -a/--archive: enables -c -m -M (no -s, which slows transfers)
- -n/--dry-run: scan + print without connecting or transferring
- -p <port>: custom SSH port (passed as -p to ssh via execvp)
- --exclude <pattern>: glob-based filename filtering in scanner
- --delete: sender collects file manifest; receiver deletes unlisted files

Protocol: added STATUS_MANIFEST, use_delete field in Config wire format.
New utilities: glob_match(), delete_extras() with recursive directory walk.
Scanner: accepts exclude patterns; skips matching entries.
SSH transport: switched from execlp to execvp for dynamic port arg.
Server + multiprocessing: handle STATUS_MANIFEST in both single and
multithreaded receive paths.

Builds clean, all 7 tests pass.

src/shared/config.h

@@ -19,10 +19,15 @@ typedef struct Config {
   bool use_sendfile;
   bool use_metadata;
   bool show_progress;
+  bool dry_run;
+  bool use_delete;
   int compression_level;
   unsigned long long chunk_size;
+  int ssh_port;
   TransportType transport;
   char *ssh_destination;
+  char **exclude_patterns;
+  int exclude_count;
 } Config;
 
 #define DEFAULT_CHUNK_SIZE (10 * 1024 * 1024)