Fixes#52
- Replace eval() in calculator() with _safe_eval() that walks the AST
and only permits: numeric constants, arithmetic ops (+,-,*,/,//,%,**),
unary +/-, math module access, and whitelisted builtins (abs, round,
min, max)
- Reject all other syntax: imports, attribute access on non-math objects,
lambdas, comprehensions, string literals, etc.
- Add 39 tests covering arithmetic, precedence, math functions,
allowed builtins, error handling, and 14 injection prevention cases
