feat(delete): per-directory delete plans for --delete-during/--delete-delay (protocol 2.24.0)

Stream one delete plan per source directory from sender to receiver instead of
a single whole-tree keep-set manifest:

- --delete-during applies each directory's extras as its plan arrives, before
  that directory's data (rsync's generator-order deletion).
- --delete-delay snapshots each directory's extras while the plan arrives and
  commits the removals only after a fully-successful transfer, so files created
  after the scan survive (matching rsync's delete-delay, not delete-after).
- Type conflicts (a destination file blocking a source directory, or vice
  versa) are cleared immediately in both modes, so the nested write succeeds.

The plan carries the destination-relative directory, its kept child directory
names and its kept child file names; the first frame also carries the global
protected prefixes, size-skipped prefixes and --delete-missing-args paths.
--delete-before keeps the existing whole-tree early manifest; plain --delete and
--delete-after keep the end-of-transfer manifest commit.

Preserves the existing safety surface: protected/size-skipped prefixes and the
--delay-updates/basis skips are honored at any depth, deletion is scoped to the
synchronized directories (--files-from), MAX_SERVER_DELETE_COUNT and
--max-delete (partial + exit 25) are shared across plans, symlinks are never
followed, and paths are confined to the receive root.

src/shared/config.c

@@ -228,7 +228,13 @@ Config* config_create(void) {
 bool config_delete_timing_early(const Config* config) {
   if (!config)
     return false;
-  return config->delete_before || config->delete_during;
+  return config->delete_before;
+}
+
+bool config_delete_timing_per_dir(const Config* config) {
+  if (!config)
+    return false;
+  return config->delete_during || config->delete_delay;
 }
 
 /* A delete-timing flag is only meaningful together with --delete.  At most one