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.

tests/test_server.c

@@ -566,7 +566,7 @@ static Config* make_late_delete_config(const char* root) {
 
 static int run_pending_receiver(Config* cfg, int fd, DeleteManifest** pending) {
   ReceiverSink sink = {0};
-  return receiver_process_pending(cfg, fd, &sink, pending);
+  return receiver_process_pending(cfg, fd, &sink, pending, NULL);
 }
 
 static void test_late_manifest_abort_frees_keepset() {
@@ -797,7 +797,7 @@ static void test_receiver_pending_commits_missing_args() {
   /* NULL pending: the single-threaded commit path deletes at FINISHED.  The
      sink sends the terminal STATUS_OK success frame. */
   ReceiverSink sink = {.send_success = true};
-  EXPECT_EQ_INT(receiver_process_pending(cfg, p[0], &sink, NULL), 0);
+  EXPECT_EQ_INT(receiver_process_pending(cfg, p[0], &sink, NULL, NULL), 0);
   Status ack;
   EXPECT_TRUE(receive_status(p[1], &ack));
   EXPECT_EQ_INT(ack, STATUS_OK);