docs(memory): note errand C=9 / distractor_idx collision is safe only single-agent

proteus/game/runtime/memory.py

@@ -247,6 +247,10 @@ def memory_frames(
                     grid[y][x] = idx
 
     distractor_idx = 9  # COLOR_MAP blue
+    # NB: errand's "C"=9 coincides with distractor_idx (9). Fine while errand is
+    # single-agent (every agent is is_chosen, so the distractor branch is unused);
+    # a future multi-agent errand would need distractor_idx to diverge or the focal
+    # and distractor couriers would be indistinguishable.
     for mt in checkpoint.memory_turns:
         if mt.agents:
             grid = [[bg] * w for _ in range(h)]