fix: preserve full calc-02 stack while limiting display to top 4

Keep the calculator stack unlimited in the demo and only constrain the rendered stack view. Also restore the edit cursor position when pulling a value into the input from the stack.

.memory/project.md

@@ -2,6 +2,7 @@
 - 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
+- Note: keep scrolling behavior in mind for the calc-02 demo when changing the stack/UI layout
 - 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

samples/calc-02/index.js

@@ -120,9 +120,7 @@ function clearStatus() {
 }
 
 function normalizeStack() {
-	while (calc.stack.length > 4) {
-		calc.stack.shift();
-	}
+	// Demo display only shows the top 4 stack values; the calculator stack remains unlimited.
 }
 
 function getStackLine(indexFromTop) {
@@ -159,7 +157,6 @@ function updateCopyButtons() {
 }
 
 function render() {
-	normalizeStack();
 	const isPortrait = window.matchMedia('(orientation: portrait)').matches || window.innerWidth <= 860;
 	calculatorEl?.classList.toggle('portrait', isPortrait);
 	calculatorEl?.classList.toggle('landscape', !isPortrait);
@@ -520,16 +517,18 @@ leftButton.addEventListener('click', () => {
 	}
 });
 downButton.addEventListener('click', () => {
-	if (!calc.isEditing && calc.isValidIndex(0)) {
+if (!calc.isEditing && calc.isValidIndex(0)) {
 	const value = calc.stack[0];
 	calc.remove(0);
 	calc.isEditing = true;
 	calc.inputValue = calc.formatNumber(value);
+	editCursor = calc.inputValue.length;
 	render();
 	focusInput();
-	}
+}
 });
 
+
 rightButton.addEventListener('click', () => {
 	if (calc.isEditing) {
 		editCursor = Math.min(calc.inputValue.length, editCursor + 1);