Manage the KV cache disc usage from preferences and stats

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

src/engine/metal.rs

@@ -801,6 +801,12 @@ impl Executor {
         self.checkpoint_tag
     }
 
+    /// Marks what the live KV state represents when no checkpoint file is
+    /// written for it, so the next turn can still continue in memory.
+    pub(super) fn note_checkpoint_tag(&mut self, tag: [u8; 32]) {
+        self.checkpoint_tag = tag;
+    }
+
     fn encode_token(&mut self, token: u32) -> Result<(), String> {
         let shape = self.model.shape;
         let map = self.model.main.map_ptr().cast();