docs: update README and memory notes

.memory/note.md

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+- README is more detailed than memory files; keep memory compact.
+- Use `state.md` for evolving engine/demo/API details.
+- `project.md` should stay as a short pointer file.

.memory/project.md

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+# Project memory
+
+- RPN calculator JS project.
+- 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.

.memory/state.md

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+# State
+- Core engine: `src/rpn-calculator.js`
+- Active demo: `samples/dev/` (HP48-style UI)
+- 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`

.memory/todo.md

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+- Keep `.memory/project.md` short.
+- Update `.memory/state.md` after project changes.
+- Reconcile README and demo docs if behavior changes.
+- Track future work around keyboard UX, stack selection, and command docs.