feat: implement rsync delete timing (--delete-before/--delete-during/--delete-delay/--delete-after)

Deletion timing is now real and selected by the four rsync flags plus the
plain --delete default. Wire protocol bumps to 2.8.0: two new config
booleans (delete_during, delete_delay) are serialized and validated, joining
the existing delete_before/delete_after.

- Early modes (--delete-before, --delete-during/--del): the sender pre-scans
  the whole tree (paths only), transmits the keep-set manifest BEFORE any
  file data, and the receiver removes extras and acks STATUS_OK; the sender
  only streams data after the deletion committed. Deletion is thus performed
  even if a later transfer phase fails (rsync delete-before/during are
  destructive by definition). FastSync streams in a single scan so it cannot
  interleave per-directory like rsync delete-during; --delete-during selects
  the same engine mode as --delete-before (documented divergence).
- Late/commit modes (plain --delete, --delete-after, --delete-delay): the
  manifest closes the data stream and deletion is committed only after
  STATUS_FINISHED proves the whole transfer succeeded, preserving FastSync's
  commit-style safety. --delete-delay converges with --delete-after because
  FastSync never snapshots the destination during data flow (documented).
- The STATUS_MANIFEST frame is now self-delimiting and position-independent.
  Single-threaded receivers delete before the success frame; the -m receiver
  hands the keep-set to server.c, which commits the deletion only after the
  disk writer thread has drained (fixes a delete-vs-in-flight-temp race).
- Every timing flag implies --delete; at most one timing flag is allowed.
- Each timing flag implies --delete, matching rsync; conflicts are rejected.

src/server/receiver.h

@@ -35,6 +35,14 @@ void receiver_outcomes_destroy(ReceiverOutcomes* outcomes);
 bool receiver_send_final_success(int fd, const Config* config, const ReceiverOutcomes* outcomes);
 
 int receiver_process(Config* config, int file_descriptor, const ReceiverSink* sink);
+/* receiver_process with an escape hatch for the commit-style (late) deletion:
+   when `pending_manifest` is non-NULL the receiver does NOT delete at
+   STATUS_FINISHED itself; instead it stores the owned keep-set manifest there
+   (leaving *pending_manifest untouched on early modes/errors) so the caller can
+   commit the deletion only after its disk writer has fully drained.  Pass NULL
+   to keep the default behaviour (delete before the success frame). */
+int receiver_process_pending(Config* config, int file_descriptor, const ReceiverSink* sink,
+                             ArrayList** pending_manifest);
 int receiver_receive_files(Config* config, int file_descriptor);
 
 #endif