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_client_cli.c

@@ -1287,6 +1287,27 @@ static void test_validate_config_delay_updates_rejects_inplace() {
   config_delete(cfg);
 }
 
+/* --backup-dir may not collide with the internal --delay-updates staging
+   directory (with or without a trailing slash), or old backups would silently
+   be installed as the "new" file. */
+static void test_validate_config_delay_updates_rejects_reserved_backup_dir() {
+  static const char* const reserved[] = {".fastsync-stage", ".fastsync-stage/"};
+  for (size_t i = 0; i < sizeof(reserved) / sizeof(reserved[0]); i++) {
+    Config* cfg = valid_client_config();
+    cfg->delay_updates = true;
+    cfg->backup_dir = str_dup(reserved[i]);
+    EXPECT_FALSE(validate_config(cfg));
+    config_delete(cfg);
+  }
+
+  /* A non-colliding backup dir is fine alongside --delay-updates. */
+  Config* ok = valid_client_config();
+  ok->delay_updates = true;
+  ok->backup_dir = str_dup("backups");
+  EXPECT_TRUE(validate_config(ok));
+  config_delete(ok);
+}
+
 void test_client_cli() {
   test_validate_config_required_paths();
   test_validate_config_incompatible_options();
@@ -1368,4 +1389,5 @@ void test_client_cli() {
   test_parse_args_temp_dir();
   test_parse_args_delay_updates();
   test_validate_config_delay_updates_rejects_inplace();
+  test_validate_config_delay_updates_rejects_reserved_backup_dir();
 }