[TSC-8] Implement Application State Manager

Add state.js ES module with in-memory state (operand1, operand2,
selectedOperation, result, error), getters/setters for each property,
and a reset() function. Update CLAUDE.md project structure listing.

CLAUDE.md

@@ -8,6 +8,7 @@ Greenfield project. Implementation is driven by the Vexa AI-SDLC pipeline (Jira
 /
 ├── index.html  # Calculator UI (semantic HTML5)
 ├── styles.css  # Responsive CSS layout and styling
+├── state.js    # In-memory application state manager (ES module)
 ├── README.md   # Project overview
 └── CLAUDE.md   # This file
 ```

state.js

@@ -0,0 +1,26 @@
+const initialState = {
+  operand1: null,
+  operand2: null,
+  selectedOperation: null,
+  result: null,
+  error: null,
+};
+
+const state = { ...initialState };
+
+export function getOperand1() { return state.operand1; }
+export function setOperand1(value) { state.operand1 = value; }
+
+export function getOperand2() { return state.operand2; }
+export function setOperand2(value) { state.operand2 = value; }
+
+export function getSelectedOperation() { return state.selectedOperation; }
+export function setSelectedOperation(value) { state.selectedOperation = value; }
+
+export function getResult() { return state.result; }
+export function setResult(value) { state.result = value; }
+
+export function getError() { return state.error; }
+export function setError(value) { state.error = value; }
+
+export function reset() { Object.assign(state, initialState); }