State

Core engine: src/rpn-calculator.js
Active demo: samples/calc-02/ responsive HP48GX layout with HP48-like X-line editing; 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