Unary negation rejected in PromQL lowering (no scalar/negate node) #36

Problem

PromQL unary negation (-expr) flips the sign of every sample — -rate(http_errors_total[5m]), -some_metric, and negation nested inside a larger expression (http_requests_total - -http_errors_total, sum(-node_cpu_seconds_total)) are all rejected. Root cause: the L2 PromQL path has no negate/scalar node to express "multiply this vector by -1", so Expr::Unary is rejected outright rather than silently computing the wrong (unnegated) result.

This is the same underlying gap as #35 (no scalar operand in the relational-level binary-op shape) — negation is effectively x * -1, which needs a scalar operand to express.

Reproduction

crates/lower/tests/promql_conformance.rs, test unary_negation_is_rejected__GAP:

#[test]
fn unary_negation_is_rejected__GAP() {
    let _ = rejected("-rate(http_errors_total[5m])");
    let _ = rejected("-some_metric");
    let _ = rejected("-metric_a or -metric_b");
    let _ = rejected("http_requests_total - -http_errors_total");
    let _ = rejected("sum(-node_cpu_seconds_total)");
}

All five currently assert rejection. Lowering them to a correctly-negated result is the acceptance criterion.