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/shared/utils.h

@@ -10,7 +10,12 @@ char* output_escape(const char* string, bool eight_bit_output);
 char* path_cat(const char* path1, const char* path2);
 bool glob_match(const char* pattern, const char* str);
 bool delete_extras(const char* dest_root, ArrayList* manifest);
-bool delete_extras_limited(const char* dest_root, ArrayList* manifest, size_t max_delete);
+/* Remove files/dirs under dest_root that are not listed in manifest.  When
+   skip_root_child is non-NULL, a direct child of dest_root with that exact
+   name is left untouched (used to protect the --delay-updates staging
+   directory, which holds files that are still to be published). */
+bool delete_extras_limited(const char* dest_root, ArrayList* manifest, size_t max_delete,
+                           const char* skip_root_child);
 bool utils_set_authorized_root(int fd, const char* canonical_path);
 /* The fd-only compatibility form is fail-closed for path-based operations;
  * callers should use utils_set_authorized_root with the canonical identity. */