Constant EVENT_MEDIA_FILE_CHANGED 

pub const EVENT_MEDIA_FILE_CHANGED: &str = "media:file_changed";

MPS event fired when a file matching a Media pane’s extension filter is created/modified inside a directory that pane is watching. Scoped per-block (block:<id>) via MuxEvent::scopes, matching EVENT_EDITOR_FILE_CHANGED’s pattern. Payload is just the changed file’s path — a wake signal, not content; the frontend re-fetches via GET /agentmux/stream-local-file.