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/utils.h

@@ -139,6 +139,11 @@ DeleteWalkResult delete_extras_limited(const char* dest_root, const ArrayList* m
                                        const DeleteSkipEntry* skips, int skip_count,
                                        size_t* deleted_out, size_t* skipped_out);
 bool delete_extras(const char* dest_root, const ArrayList* manifest);
+/* Open the existing destination directory at `dest_root`, confined to the
+   authorized root with an O_NOFOLLOW component walk (the same confinement the
+   deletion walker uses for its root).  Returns a new fd the caller owns, or -1
+   on error (including a destination that does not exist). */
+int utils_open_authorized_destination(const char* dest_root);
 bool utils_set_authorized_root(int fd, const char* canonical_path);
 /* The fd-only compatibility form is fail-closed for path-based operations;
  * callers should use utils_set_authorized_root with the canonical identity. */