Add safe expression evaluator (calc/engine/evaluator.py + builtins.py)

- evaluator.evaluate(expr, extra_names) returns (result_str, is_error)
- blocks __builtins__ to prevent arbitrary code execution
- handles ZeroDivisionError, ValueError, NameError with friendly messages
- builtins.SAFE_NAMES covers trig, hyperbolic, log, exp, rounding, constants

calc/engine/evaluator.py

@@ -0,0 +1,22 @@
+from calc.engine.builtins import SAFE_NAMES
+
+
+def evaluate(expr: str, extra_names: dict = None) -> tuple[str, bool]:
+    """
+    Evaluate a math expression safely.
+    Returns (result_string, is_error).
+    """
+    namespace = dict(SAFE_NAMES)
+    if extra_names:
+        namespace.update(extra_names)
+    try:
+        result = eval(expr, {"__builtins__": {}}, namespace)
+        return str(result), False
+    except ZeroDivisionError:
+        return "Error: Division by zero", True
+    except ValueError as err:
+        return f"Error: {err}", True
+    except NameError as err:
+        return f"Error: {err}", True
+    except Exception as err:
+        return f"Error: {err}", True