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/shared/protocol.h

@@ -191,7 +191,9 @@ enum NET_STATUS {
    * (--delete-delay).  Payload: an int32 has_config flag (1 on the first plan
    * of the run, 0 afterwards); when set, the three global config sections
    * (protected-prefix count+paths, size-skipped count+paths, missing-args
-   * count+paths); then the destination-relative directory path wire string
+   * count+paths); then an int32 apply flag (1 for a real plan, 0 for a
+   * config-only carrier frame that must not walk a directory); then the
+   * destination-relative directory path wire string
    * ("." for the receive root); then the child-directory count + names and the
    * child-file count + names that must be kept.  Appended after
    * STATUS_DEST_INFO so no existing status is renumbered. */