feat: add structured per-file change output model (-i, --list-only, --out-format, --log-file-format)

Implement rsync-style itemized output backed by one shared change-event
engine (src/client/change_list.c):
- -i/--itemize-changes prints ">f+++++++++ <path>" for files actually sent
  (single-threaded and -m); unchanged files print nothing.
- --list-only prints an ls-style listing of files that would be transferred
  without contacting the server or writing anything.
- --out-format=FORMAT prints a printf-style template per changed file
  (tokens %%f %%n %%l %%b %%M %%%%; unknown escapes preserved).
- --log-file-format=FMT logs each transferred file when --log-file is set.
Events are emitted from the per-file sender path shared by both transfer
modes, so the single sender thread is the only reporter (no races).

src/client/client_validation.c

@@ -44,6 +44,10 @@ bool validate_config(const Config* config) {
     log_message(LOG_LEVEL_ERROR, "--delta cannot be combined with -f (sendfile)");
     return false;
   }
+  if (config->log_file_format && !config->log_file) {
+    log_message(LOG_LEVEL_ERROR, "--log-file-format requires --log-file");
+    return false;
+  }
   if (config->append || config->append_verify) {
     fprintf(
         stderr,