feat(promql): range-vector functions over a sub-query argument (#55)

[zzylol]

Closes #55. Generalizes the *_over_time-over-sub-query support (#42) to the whole range-vector family: rate/irate/increase and the counter-derivatives (changes/delta/idelta/deriv/resets/predict_linear/double_exponential_smoothing).

Before / after

rate(sum(m)[5m:]), changes(rate(m[5m])[1h:]), etc. were rejected (expected a range-vector (matrix) argument). Now each lowers to a per-series Aggregate{[f]} directly over the Subquery — the sub-query is the range context, so there is no separate TimeRange.

How

• Front end (promql.rs): walk_call routes any range function whose matrix arg is a sub-query through a unified range_fn_over_subquery. rate/increase read the window from the sub-query's own range; predict_linear/double_exp thread their scalar params; quantile_over_time reads φ from arg 0.
• Converter (l2/lower.rs): a PromQLSubquery input is the range context, so (1) no TimeRange is synthesized over it (rate(sub) would otherwise get a spurious TimeRange from its carried window), and (2) the L2 hardening: counter-derivative intent can be emitted range-less if it reaches the converter without a Window #71 counter-derivative range-less guard correctly does not fire when the input is a sub-query.

Verified

rate(sum(m)[5m:]) → Aggregate{Rate} over Subquery{5m} (no double range); changes(rate(m[5m])[1h:]) → Aggregate{Changes} over Subquery{1h} over Aggregate{Rate} over TimeRange{5m}; params carried for predict_linear/double_exp. Normal matrix forms unchanged. Flips counter_derivative_over_a_subquery_is_rejected__GAP into passing conformance tests. Full suite green (29 binaries), clippy clean.

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