Plan: require class encapsulation for stateful components

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

plan.md

@@ -116,6 +116,15 @@ collisions with host app styles. The host can override via CSS custom properties
 ```
 
 
+## Code Structure
+
+All stateful components must be classes, not bare module-level variables. The
+current `app.mjs` has scanner state (`running`, `last_decoded_text`, `fps`,
+etc.) scattered as module globals — this needs to be encapsulated in
+`Barcode_Scanner` before the library refactor proceeds. Module scope is only
+for constants and exports.
+
+
 ## Lifecycle
 
 ```