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.

tests/test_server.c

@@ -180,7 +180,10 @@ static void test_receive_manifest_rejects_traversal() {
   io_set_fds(p[0], p[1]);
   EXPECT_TRUE(send_int(p[1], 1));
   EXPECT_TRUE(send_str(p[1], "../outside"));
-  EXPECT_EQ_INT(receive_manifest(p[0], cfg, NULL), -1);
+  EXPECT_NULL(receive_manifest_entries(p[0]));
+  Status status;
+  EXPECT_TRUE(receive_status(p[1], &status));
+  EXPECT_EQ_INT(status, STATUS_ERROR);
   close(p[0]);
   close(p[1]);
   config_delete(cfg);