mind: double-buffer MindState for UI diffing

UI event loop clones MindState on each render tick, diffs against
the previous copy, and generates status updates from changes. Mind
no longer sends UiMessage::StatusUpdate — state changes are detected
automatically by the UI.

Removes update_status from both Mind and event_loop. DMN state
changes, turn tracking, scoring status all flow through the diff.

Zero UiMessage sends from Mind's run loop for state changes.

Co-Authored-By: Kent Overstreet <kent.overstreet@linux.dev>

src/mind/dmn.rs

@@ -22,7 +22,7 @@ use std::path::PathBuf;
 use std::time::{Duration, Instant};
 
 /// DMN state machine.
-#[derive(Debug)]
+#[derive(Debug, Clone)]
 pub enum State {
     /// Responding to user input. Short interval — stay engaged.
     Engaged,