feat(promql): lower unary negation as multiply-by-minus-one (#36)

[zzylol]

Closes #36.

Problem

PromQL unary negation (-expr) was rejected outright: -rate(m[5m]), -some_metric, sum(-m), a - -b all errored. The root cause was that the L2 PromQL path had no scalar/negate node to express "multiply this vector by -1" — negation is effectively x * -1, which needs a scalar operand.

Fix

That scalar operand landed in #35 (QueryExpr::Scalar + scalar BinaryOp operands), so this is now a small, localized change to the one rejection site in the PromQL walker:

• Vector operand → expr * -1: a Mul BinaryOp of the (label-preserving) vector against Scalar(-1), no vector match. Mul is commutative, so operand order carries no hazard, and the existing L3 BinaryOp schema rule already follows the non-scalar side — so negation is label-preserving with no extra handling.
• Constant-foldable operand (-(10*1024*1024)) → a negated Scalar leaf, matching how bare literals already fold.

Because the aggregate and binary walkers already recurse through the shared walk (from the #27 nesting work), negation composes everywhere for free: inside aggregate arguments (sum(-m)), binary/set ops (a - -b, -a or -b), and nested (- -m).

Tests

The five cases the old unary_negation_is_rejected__GAP test pinned as rejected now lower correctly. That test is replaced by:

• unary_negation_lowers_as_multiply_by_minus_one — all five original cases lower; the root -some_metric is checked structurally (Scan * Scalar(-1), no vector match, [ts, value] schema preserved), and sum(-m) is verified to nest the negation under the aggregate.
• unary_negation_of_constant_folds_to_scalar and double_unary_negation_nests — the fold and nesting edge cases.
• Two exact-tree e2e pins in binary_op.rs (q36_unary_negation_is_multiply_by_minus_one, q36_sum_of_negation_nests).
• Moved unary negation from "Rejected cleanly" to "Lowers" in the docs/promql-lowering.md nesting-contract table; scrubbed the two stale "negation rejected" references in promql.rs / error.rs.

The full PromQL corpus suite (awesome-prometheus-alerts + promql corpora) still passes — no rejection-count assertion shifted.

Notes

• Front-end crate only; independent of any other open work.
• Verified: cargo test --workspace — 340 passing, 0 failures; cargo clippy --all-targets clean; production source (promql.rs, error.rs) has zero net-new cargo fmt drift vs main.

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