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/usage.c

@@ -81,6 +81,10 @@ void print_usage(void) {
   printf("  --backup-dir <dir>  Directory for backups (requires --backup)\n");
   printf("  --suffix <str>      Backup suffix (default: ~)\n");
   printf("  --stats             Print transfer statistics at end\n");
+  printf("  -i, --itemize-changes  Print an rsync-style per-file change line\n");
+  printf("  --out-format=FORMAT Output format for changed files (%%f %%n %%l %%b %%M %%%%)\n");
+  printf("  --list-only         List source files instead of transferring\n");
+  printf("  --log-file-format=FORMAT  Per-file log line format (needs --log-file)\n");
   printf("  -h, --human-readable  Print byte sizes in human-readable form\n");
   printf("  --max-depth <n>     Maximum directory depth (0=unlimited)\n");
   printf("  --log-file <path>   Write log messages to file\n");