feat(delete): default --delete to rsync delete-during; add --delete-commit

- plain --delete with no timing flag now selects delete-during (progressive
  deletion, matching rsync and avoiding the full old+new tree peak)
- new long-only FastSync --delete-commit restores the old atomic behavior
  (delete only after the whole transfer succeeds); timing-identical to
  --delete-after, implemented via the same wire bool
- timing flags are mutually exclusive; --delete-commit conflicts with other
  timings; --delete-before/--delete-during rows reworded per Phase-0 probes
- CHANGELOG migration note; tally unchanged 116/14/27

src/client/client_send.c

@@ -3287,7 +3287,8 @@ int send_files(Config* config) {
       prepared.options.synced_dirs = synced_dirs;
     }
   }
-  /* The late-timing modes (plain --delete / --delete-after) build the manifest
+  /* The late-timing modes (--delete-after/--delete-commit and a plain --delete
+     that fell back from per-dir mode because of -d/--dirs) build the manifest
      while streaming and send it after the last data frame.  --delete-before
      sends a whole-tree keep-set up front; --delete-during/--delete-delay build a
      per-directory plan set up front (paths only) and stream the plans alongside