From the control channel (an agent, `mcp --connect`) the command ran in
Session::execute on the UI thread and stat-ed every photo's file, which on
a large library on a network share froze the window for minutes. The app
now checks the files on a worker thread under a "Missing Photos" row of the
activity stack (counted, with ✕), answers the request with the same
[{id, path}] when the check ends, lets a second request share the running
check, and hands an answer nobody was waiting for to the next request.
The command itself asks the session's existence check, the one the
availability cache uses.
Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>
