PromQL: agg intents should carry an explicit col, not implicit None for the value column #130

`AggIntent::{Sum,Avg,Quantile,Cardinality,...}.col` is `Option`. SQL uses it literally — `Some(idx)` names the reduced column, `None` means no column applies (e.g. `COUNT(*)`). PromQL instead always emits `col: None` to mean "the implicit sample value column" (`crates/l2/src/lower.rs:835`), a documented convention rather than something the field states.

This overloads `None` with two different meanings depending on source language, so `AggIntent` alone doesn't tell you which case you're in — a reader has to know the per-language convention.