State

Core engine: src/rpn-calculator.js
Reference demo: samples/calc-02/ (portrait-first HP48GX layout, compact mode/constants popups; Const button comes before Mode in the display row)
Important UI behavior: mode button shows the current angle mode; keyboard focus stays on the hidden input on desktop; clipboard paste is supported
Public API: push, pop, clear, swap, remove, edit, isValidIndex, input, command, getOperationsByCategory, getConstants, listConstants, setConstant, removeConstant, hasConstant
Config: maxSize, base, angleMode, enabledCommands
Commands: arithmetic, stack, trigonometry, constants pi and e; arithmetic includes root; constants can be added or removed dynamically through the core API
Exports: browser window.RpnCalculator, CommonJS module.exports