Fix buffered processing log typing sequence

doc/frontend-design-foundation.md

@@ -42,6 +42,13 @@ Do not hardcode new palette or spacing values in component styles when a token a
 - Keep transitions brief and functional.
 - Avoid decorative animation loops outside explicit status indicators like terminal caret blink.
 
+## Processing Log Timeline Behavior
+
+- Keep processing log headers in strict arrival order when typing animation is enabled.
+- Buffer newly discovered entries and reveal only one active line at a time while it types.
+- Do not render queued headers before their animation starts, even when polling returns batched updates.
+- Preserve existing header content format and fold/unfold detail behavior as lines are revealed.
+
 ## Extension Checklist
 
 When adding or redesigning a UI area: