feat(lower): arbitrary PromQL function nesting in aggregates (#27)

[zzylol]

What & why

Supports nested query functions in PromQL — an aggregate over a composite argument — closing the core PromQL part of #27.

The L3 intent algebra, the L2→L3 converter, and the Binder are already fully recursive; the only ceiling was the PromQL front end. walk_aggregate (crates/lower/src/promql.rs) lowered an aggregate's argument through a flat two-level template (Inner/Outer): an outer aggregator over exactly one inner selector or range-vector function. That rejected the common cases:

• max(sum by (job) (rate(m[5m]))) — outer aggregate over a nested aggregate
• topk(3, sum by (instance) (rate(m[5m]))) — was pinned as topk_over_aggregate_arg_is_rejected__GAP
• sum(rate(a[5m]) + rate(b[5m])) — aggregate over a binary op
• sum(histogram_quantile(0.9, sum by (le) (rate(h_bucket[5m]))))

How

• walk_aggregate tries the flat fast path first (lower_inner), preserving heavy-hitter topk(k, count_over_time(...)) recognition. When the argument is a composite expression it recurses via the same walk used at the top level and wraps the result in the outer op (outer_kind + build_over_subtree). Unsupported inner expressions (e.g. unary negation) still surface their error — nothing is silently mislowered.
• ColumnRef::SampleValue resolution (column_resolution.rs) now also resolves to the sole numeric non-timestamp column, so an outer ranking over a cross-series aggregate (value column renamed sum) finds its sort key.

No changes to the L3 IR, the converter, or the Binder were needed — only the front end and one resolver fallback.

Tests

• Flips topk_over_aggregate_arg_is_rejected__GAP → topk_over_nested_aggregate_is_generic_sort_limit (passing).
• Adds outer_aggregate_over_nested_aggregate_nests, aggregate_over_binary_op_nests (conformance), q27_max_over_sum_by_job_over_rate (exact-tree e2e), and two SampleValue resolution unit tests.
• Full workspace test suite green; cargo clippy --all-targets clean.

Scope / follow-ups

This PR covers PromQL aggregate nesting. Remaining tasks (range-vector-function-over-subquery, topk(k, <bare selector>) by (...), SQL derived tables/inline views) are enumerated in the updated #27.

Closes part of #27.

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