nx_probe_opstart_live.nx

nx_probe_opstart_live.nx -- MUST-REFUSE WITNESS for the operator-at-expression-start refusal (parse_primary denylist: "an operator starts this expression"), written 2026-09-02 because its old witness, nx_probe_mlop_live.nx, stopped reaching the condition when LN15 shipped: a line opening with `+` now JOINS the expression above it, and that program is simply correct. THIS fixture cannot be joined into anything correct. After `a +` the parser asks parse_unary for an operand; `+` has no prefix form (parse_unary consumes - ! ~ & * only), so parse_primary is handed an operator token with the expression already open. The rule did not change; only its irreducible witness did. EXPECT: COMPILE-FAIL, exit 2, with 'an operator starts this expression'. If this ever BUILDS, the parser-desync refusal has been lost -- there is no benign reading of `a + + b`.