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/builtins.py

@@ -0,0 +1,26 @@
+import math
+
+SAFE_NAMES = {
+    # trig (radians)
+    "sin": math.sin, "cos": math.cos, "tan": math.tan,
+    "asin": math.asin, "acos": math.acos, "atan": math.atan,
+    "atan2": math.atan2,
+    # trig (degrees)
+    "sind": lambda x: math.sin(math.radians(x)),
+    "cosd": lambda x: math.cos(math.radians(x)),
+    "tand": lambda x: math.tan(math.radians(x)),
+    # hyperbolic
+    "sinh": math.sinh, "cosh": math.cosh, "tanh": math.tanh,
+    # logs & exp
+    "log": math.log, "log2": math.log2, "log10": math.log10,
+    "ln": math.log, "exp": math.exp,
+    # powers & roots
+    "sqrt": math.sqrt, "pow": math.pow,
+    # rounding
+    "abs": abs, "round": round, "floor": math.floor, "ceil": math.ceil,
+    # misc
+    "factorial": math.factorial, "gcd": math.gcd,
+    "degrees": math.degrees, "radians": math.radians,
+    # constants
+    "pi": math.pi, "e": math.e, "tau": math.tau, "inf": math.inf,
+}