feat(p7-times): real directory/symlink time preservation; -O/-J meaningful

Wave D of Phase 7. Make -O/--omit-dir-times and -J/--omit-link-times real by
preserving directory and symlink times, and mark --secluded-args as an explicit
Impossible/Divergence no-op.

Wire: PROTOCOL_VERSION 2.16.0 -> 2.17.0. Adds a terminal STATUS_DIR_TIMES frame
(int count + (wire path, metadata) pairs) sent after all file data and the
optional delete manifest. STATUS_MKDIR also carries metadata for --dirs entries.
Config-frame layout is unchanged.

Sender: the recursive scanner captures every traversed source directory (both
DirectoryScanner and the parallel scanner root + workers, appends mutex-guarded)
into a shared list; the single-threaded and -m paths transmit it last.

Receiver: a DirTimeList accumulates received directory metadata and applies it
with fd-relative no-follow utimensat only at the very end -- after all children,
after the commit-style --delete, and after --delay-updates publication -- in the
single-threaded success frame and in server.c after the -m threads join. -O skips
the application. Symlink metadata is applied at link creation with
utimensat/fchownat/fchmodat AT_SYMLINK_NOFOLLOW; -J suppresses only link times.
identity_apply_ownership_link shares the identity resolver with the fd path.

Docs: -O/-J rows -> Implemented; --secluded-args -> Impossible/Divergence;
--protocol accepted/rejected values and Phase-6/7 notes updated.

Tests: unit (scanner dir capture, DirTimeList apply, symlink metadata, protocol
version values) and integration (dir mtime round-trip + -O, symlink mtime
round-trip + -J, independent suppression), parameterized over single/multithread.

src/shared/protocol.h

@@ -103,7 +103,16 @@ enum NET_STATUS {
    * int32 rdev major/minor fields.  The receiver validates the kind and rdev,
    * confines the node below the receive root, and recreates it (mknod/mkfifo),
    * privilege-gating the mknod.  Protocol 2.13.0. */
-  STATUS_SPECIAL
+  STATUS_SPECIAL,
+  /* Directory-time superstructure (P7 Wave D, protocol 2.17.0): a single
+   * terminal frame sent after all file data (and after the optional delete
+   * manifest) carrying every source directory's captured metadata so the
+   * receiver can apply directory mtimes/atimes AFTER all of a directory's
+   * children have been written.  Payload: an int count, then count repetitions
+   * of (wire path string, metadata frame).  The receiver defers the actual
+   * utimensat until its own delete/publish phase has committed, then skips the
+   * whole set when -O/--omit-dir-times is set. */
+  STATUS_DIR_TIMES
 };
 
 void io_set_fds(int read_fd, int write_fd);