# State
- Core engine: `src/rpn-calculator.js`
- Active demo: `samples/calc-02/` responsive HP48GX layout driven by portrait/landscape text mockups; display-adjacent button row stays in 4 columns
- Mode button shows the current angle mode only; selecting a mode uses a popup menu
- Public API: `push`, `pop`, `clear`, `swap`, `remove`, `edit`, `isValidIndex`, `input`, `command`, `getOperationsByCategory`, `getConstants`
- Config: `maxSize`, `base`, `angleMode`, `enabledCommands`
- Commands: arithmetic, stack, trigonometry, constants `pi` and `e`
- Exports: browser `window.RpnCalculator`, CommonJS `module.exports`