[TSC-10] Implement calculation engine with arithmetic operations

Add calculator.js with a pure calculate(a, b, operation) engine returning
{result} or {error}, wired to the existing HTML UI via an IIFE. Update
index.html to load the script and CLAUDE.md to document the new module.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

CLAUDE.md

@@ -6,10 +6,11 @@ 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
-├── README.md   # Project overview
-└── CLAUDE.md   # This file
+├── index.html      # Calculator UI (semantic HTML5)
+├── styles.css      # Responsive CSS layout and styling
+├── calculator.js   # Calculation engine + UI wiring (plain JS, no build step)
+├── README.md       # Project overview
+└── CLAUDE.md       # This file
 ```
 
 ## Build Commands
@@ -29,6 +30,7 @@ No dev server or REPL is configured yet. Update this section once the implementa
 - **Domain**: Simple Calculator — arithmetic operations (add, subtract, multiply, divide at minimum).
 - **Stack**: Plain HTML/CSS/JS — no framework or build step.
 - `index.html` contains the full UI structure: operand inputs, operation buttons, calculate/clear buttons, result `<output>`, and an error `<p role="alert">`.
+- `calculator.js` exposes a `calculate(a, b, operation)` function returning `{ result }` or `{ error }`, and an IIFE that wires the engine to the DOM.
 
 ## Code Style
 

calculator.js

@@ -0,0 +1,62 @@
+function calculate(a, b, operation) {
+  switch (operation) {
+    case 'add':      return { result: a + b };
+    case 'subtract': return { result: a - b };
+    case 'multiply': return { result: a * b };
+    case 'divide':
+      if (b === 0) return { error: 'Division by zero is not allowed.' };
+      return { result: a / b };
+    default:
+      return { error: 'Unknown operation: ' + operation };
+  }
+}
+
+(function () {
+  var form = document.getElementById('calculator-form');
+  var operationBtns = document.querySelectorAll('button[name="operation"]');
+  var resultEl = document.getElementById('result');
+  var errorEl = document.getElementById('error-message');
+  var selectedOperation = null;
+
+  operationBtns.forEach(function (btn) {
+    btn.addEventListener('click', function () {
+      operationBtns.forEach(function (b) { b.setAttribute('aria-pressed', 'false'); });
+      btn.setAttribute('aria-pressed', 'true');
+      selectedOperation = btn.value;
+    });
+  });
+
+  form.addEventListener('submit', function (e) {
+    e.preventDefault();
+    resultEl.textContent = '';
+    errorEl.textContent = '';
+
+    var a = parseFloat(document.getElementById('operand1').value);
+    var b = parseFloat(document.getElementById('operand2').value);
+
+    if (isNaN(a) || isNaN(b)) {
+      errorEl.textContent = 'Please enter valid numbers for both operands.';
+      return;
+    }
+
+    if (!selectedOperation) {
+      errorEl.textContent = 'Please select an operation.';
+      return;
+    }
+
+    var outcome = calculate(a, b, selectedOperation);
+
+    if (outcome.error) {
+      errorEl.textContent = outcome.error;
+    } else {
+      resultEl.textContent = outcome.result;
+    }
+  });
+
+  form.addEventListener('reset', function () {
+    resultEl.textContent = '';
+    errorEl.textContent = '';
+    operationBtns.forEach(function (btn) { btn.setAttribute('aria-pressed', 'false'); });
+    selectedOperation = null;
+  });
+}());

index.html

@@ -51,5 +51,6 @@
       <p id="error-message" role="alert" aria-label="Error message"></p>
     </section>
   </main>
+  <script src="calculator.js"></script>
 </body>
 </html>