BACKLOG-eval

Build backlog

D1 arithmetic — evaluate() correct for +/-/*//, precedence, parens, unary minus
D2 division — true division; EvalError on zero; not bare ZeroDivisionError
D3 result type — whole floats → int; non-whole → float; rule documented in evaluator.py
D4 CLI — calc.py at work root; stdout+exit-0 on success; stderr+exit-1 on error; no traceback
D5 tests — 24 new tests in calc/test_evaluator.py; 68 total pass; prior suite unaffected

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