feat(shared): --delete-missing-args config/wire, manifest third section, receiver exact-path deletions

PROTOCOL_VERSION 2.9.0 -> 2.10.0. The STATUS_MANIFEST frame gains a third
section carrying destination-relative exact-delete paths (the missing
--files-from entries' mirrors); the config frame gains a delete_missing_args
bool (ignore_missing_args stays client-only). The receiver validates the third
section like the keep-set and commits it with manifest_delete_all():
manifest_delete_missing_args runs first (explicit user requests, never blocked
by protected-prefix exclusion protection; staging/basis protected; a non-empty
directory mirror removed only under --force/--delete, rsync parity) and then the
ordinary extras walk. Server --allow-delete gates it like --delete.

src/shared/utils.c

@@ -208,8 +208,8 @@ static bool is_dir_in_manifest(const char* rel_path, ArrayList* manifest) {
    therefore protects "a" and "a/b/c" but not "ab".  Entries with top_level_only
    set only protect DIRECT children of the receive root (at_root); nested
    directories that share such a name stay ordinary destination content. */
-static bool path_under_skip_prefix(const char* child_rel, bool at_root,
-                                   const DeleteSkipEntry* skips, int skip_count) {
+bool path_under_skip_prefix(const char* child_rel, bool at_root, const DeleteSkipEntry* skips,
+                            int skip_count) {
   for (int i = 0; i < skip_count; i++) {
     if (skips[i].top_level_only && !at_root)
       continue;