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.

src/client/client_validation.c

@@ -1,4 +1,5 @@
 #include "client_validation.h"
+#include "delay_updates.h"
 #include "log.h"
 #include "usage.h"
 #include <stdio.h>
@@ -64,5 +65,11 @@ bool validate_config(const Config* config) {
     log_message(LOG_LEVEL_ERROR, "--delay-updates does not work with --inplace");
     return false;
   }
+  if (config->delay_updates && delay_updates_staging_name_conflict(config->backup_dir)) {
+    log_message(LOG_LEVEL_ERROR,
+                "--backup-dir is reserved when --delay-updates is active (used for the internal "
+                "staging directory)");
+    return false;
+  }
   return true;
 }