Give each signature snapshot a private child-resource scope. Its composite
root lease closes those children before removing the directory and retains
both stages across failed retries. Bind local cleanup failures to their
exact owner so child release cannot clear a failed root removal.
Replace the mixed-outcome regression that expected premature root removal.
Cover real descriptor denial, primary/cancellation preservation, MCP close
and same-connection recovery. Expose underlying real codesign test errors
before asserting result fields.
Fixes#5052
morluto
9e259f87c8