feat: add root command and fix calc-02 exponent shortcuts

.memory/project.md

@@ -4,3 +4,5 @@
 - Read `.memory/state.md` for current state.
 - Keep names and commands in English.
 - Update memory files based on events: engine, demo, API, commands, exports, docs, or tasks.
+- Core arithmetic now includes `root` for y-th roots, and `samples/calc-02/` uses it for `y√x`.
+- `samples/calc-02/` in portrait mode remains the active responsive demo.

.memory/state.md

@@ -1,9 +1,9 @@
 # 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
+- Active demo: `samples/calc-02/` responsive HP48GX layout with portrait/landscape support; 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`
+- Commands: arithmetic, stack, trigonometry, constants `pi` and `e`; arithmetic now includes `root` for y-th roots
 - Demo actions: keyboard focus is kept on the hidden input on desktop so typing keeps working; the keypad layout places Enter in the bottom-left, ± in the former Enter position, and Esc before Clear for safety; paste parses clipboard text as a number before pushing it to the stack; Ctrl+V is supported via the hidden input paste event; backspace is ignored when the stack is empty; operation errors are shown as an overlay bar on top of the calculator with a shorter timeout and darker red
 - Exports: browser `window.RpnCalculator`, CommonJS `module.exports`