lowerBinaryOp derived the result type from the LHS alone and emitted
add/sub/mul/div/mod without checking the RHS, so `s64 + string` lowered
as `add : s64` and reinterpreted the string's bytes — printing garbage
instead of erroring.
Add isArithOperand (int / float / vector / pointer, plus custom int
widths) and, for `+ - * / %`, diagnose `cannot apply '<op>' to operands
of type '<lhs>' and '<rhs>'` and return a placeholder sentinel instead of
the corrupting op. `.unresolved` operands pass through so a type we
couldn't infer is never falsely rejected; the existing optional-unwrap
and int×float promotion are accounted for before the check.
Ordering (`< <= > >=`) and bitwise/shift (`& | ^ << >>`) ops share the
same LHS-derived-type hole and are left as a noted follow-up in the issue.
Regression: examples/214-arith-operand-type-check.sx (s64 + string, and
non-numeric LHS string * s64).
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