fix: PromQL stddev/stdvar should lower to population=true

[milindsrivastava1997]

PromQL defines stddev, stdvar, stddev_over_time, and stdvar_over_time as population statistics (divide by N).

The current lowering in crates/lower/src/promql.rs maps all four to population: false (sample stddev, divide by N-1):

InnerFunc::StdDev => AggFunc::StdDev { population: false },
InnerFunc::Variance => AggFunc::Variance { population: false },
// and outer_func maps the same way

These should be population: true. The e2e tests in tier2_agg.rs and tier3_range.rs currently assert population: false to match the lowering — they should be updated to population: true once this is fixed.