refactor(intent_algebra): retire SketchAgg/WindowedAgg — legacy_expr is a pure L2 IR - #229

…is now a pure L2 IR

PR #228 folded the `legacy_lower` sketch-fusion pass into `convert_root`,
but the sketch-fused legacy Layer-3 IR (`SketchAgg` / `WindowedAgg`) still
existed as the intermediate between two internal walks. This collapses
that to a single walk and deletes the intermediate.

`convert`'s `Aggregate` arm now does the single-statistic sketchable
fusion directly in canonical terms — `Window`-input → `Window { Aggregate
{ by: [] } }`, `GROUP BY` keys → wrapping `Partition`, `StdDev` /
`Variance` → `Merge` of sibling quantile aggregates — instead of emitting
intermediate `SketchAgg` / `WindowedAgg` nodes for the (now-deleted)
second walk to un-fold. `convert_root` is just `Binder::bind` + `convert`.

With the fusion done in canonical terms, the legacy IR has no remaining
producer or consumer of the fused variants, so they're removed:

  * `legacy_expr::QueryExpr`: `SketchAgg` and `WindowedAgg` variants gone;
    `WindowSpec` / `WindowKind` (only `WindowedAgg` used them) gone.
    `legacy_expr` is now a pure Layer-2 *relational* IR.
  * `legacy_to_canonical`: the `SketchAgg` / `WindowedAgg` convert arms,
    `lower_to_sketch_algebra`, `lower_aggregate`, `map_window_kind`, and
    the `lower_tests` module are deleted.
  * `binder.rs`: `collect_referenced_columns` drops the
    `SketchAgg`/`WindowedAgg` arm — the converter never resolved
    `AggItem.col` positionally anyway, only group-by keys.

Behavior is unchanged for every tree the parsers emit: the merged
`Aggregate` arm matches the *raw* `Window` input, which is exact because
the parsers never nest `Aggregate` directly over `Window` directly over
another sketchable `Aggregate` — the only shape where raw vs.
sketch-lowered input would diverge.

`window_fusion.rs` tests, which built legacy `WindowedAgg` nodes by hand,
are rewritten to build the canonical `Window { Aggregate }` shape directly
(or L2 `Aggregate { Window }` + `convert_root`). design.md §3/§6 stale
`legacy_lower` / fused-L3 references refreshed.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>