fix: --delay-updates delete/backup collisions, publish-failure test, staging lock

Review fixes for --delay-updates:

- --delete no longer deletes the staged files: the delete walker gains a
  skip_root_child parameter and receive_manifest passes DELAY_UPDATES_STAGING_DIR
  when delay_updates is active, so deletion removes genuine extras while the
  staging dir (a direct child of the receive root) is left for publication in
  both single and -m modes.
- --backup-dir is rejected when it collides with the reserved internal staging
  name .fastsync-stage (trailing slash normalized), in client validation and in
  the received-config wire validation, preventing old backups from being
  silently installed as new files.
- Staging dir is now held under an exclusive advisory flock for the whole
  transfer (context lifetime): two simultaneous delayed transfers to one
  destination root no longer share/destroy each other's staged data - the
  second fails cleanly.  Cleanup only touches the staging dir when this context
  owns the lock, so a lock-contention failure cannot wipe a live session.
- Post-publish staging cleanup now returns/logs instead of discarding failures
  (warning when the staging dir cannot be fully removed).
- Reworked the publish-failure integration test to exercise real mid-publish
  semantics (top-level file published, nested rename fails, no rollback,
  sources retained under --remove-source-files) and added integration tests for
  --delete + --delay-updates ordering and reserved --backup-dir rejection.
- RSYNC_COMPAT note documents delete ordering, the reserved-name hazard, and
  the concurrency guard.

tests/test_delay_updates.c

@@ -274,7 +274,20 @@ out:
   remove_tree(root);
 }
 
+/* The reserved staging name must be recognizable for validation, including
+   with a trailing slash. */
+static void test_delay_updates_reserved_name_helper() {
+  EXPECT_TRUE(delay_updates_staging_name_conflict(".fastsync-stage"));
+  EXPECT_TRUE(delay_updates_staging_name_conflict(".fastsync-stage/"));
+  EXPECT_TRUE(delay_updates_staging_name_conflict(".fastsync-stage///"));
+  EXPECT_FALSE(delay_updates_staging_name_conflict(NULL));
+  EXPECT_FALSE(delay_updates_staging_name_conflict(""));
+  EXPECT_FALSE(delay_updates_staging_name_conflict("backups"));
+  EXPECT_FALSE(delay_updates_staging_name_conflict(".fastsync-stage.bak"));
+}
+
 void test_delay_updates() {
+  test_delay_updates_reserved_name_helper();
   test_delay_updates_no_final_before_publish();
   test_delay_updates_publish_installs_files();
   test_delay_updates_cleanup_removes_staged();