refactor(legacy_expr): batch 1 — D-purge + Dedup→Distinct rename

[zzylol]

Summary

Lowest-risk first batch of the legacy_expr retirement plan (the Plan agent's 13-PR sequence). Deletes 10 unused variants + does one structural rename. Lays the groundwork for Batch 2 (A-lifts into canonical L3) and the heavier C-reshape PRs that follow.

What landed

Deleted: 5 QueryExpr variants (0 constructor sites)

Variant	Why deleted
JoinSketch { join_key, outer, inner }	Sketch-bound physical alternative; design.md says L3 must be intent-only; sketch-bound shape lives at L4 (sketch_algebra::SketchExpr::SketchJoin)
Subquery { alias, expr }	Designed for SQL nested SELECT, no parser emits it
WindowFunc { …, frame, … }	Designed for SQL OVER(), no parser emits it
SketchCoverage	Internal enum; referenced only by its own test
(Dedup→Distinct rename in step 2)	—

Deleted: 5 ScalarExpr variants (0 constructor sites)

Variant	Notes
UnaryOp { op, input }	—
InSubquery { …, subquery, … }	—
Case { operand, when_then, else_ }	—
Cast { expr, to }	—
VectorBinaryOp { …, vector_match }	design.md merges PromQL vector ops into QueryExpr::BinaryOp { vector_match }

Orphaned supporting types also dropped

UnaryOpKind, WindowFuncKind, WindowFrame, FrameUnit, FrameBound, legacy DataType. All had no consumers after the D-purge. The canonical intent_algebra::schema::DataType is untouched.

Dedup → Distinct rename + N-arity

QueryExpr::Dedup { col: ColumnRef } → QueryExpr::Distinct { cols: Vec<ColumnRef> }. Per design.md §6 "What was removed" row 7.

Touched: enum decl, walk/source_name arms, query_parser/mod.rs, physical/stage_split.rs (×2), physical/planner.rs (new display_distinct_cols helper), physical/allocator.rs (incl. routing doc), optimizer/engine.rs (cost factor, deployment-stage map, HLLDedupElim pattern, recursive rewriter, 2 tests), intent_algebra/legacy_lower.rs. HLLDedupElim rule name kept (still semantically accurate).

Canonical AggIntent already clean

intent_algebra/agg_intent.rs has no HistogramQuantile (removed in #133) and no PromQLSubquery. No change needed.

Build + test

• cargo build --release -p controller — clean
• cargo build --release -p query_engine_rust — clean
• cargo test -p controller --lib — 655 passed, 0 failed (was 656/656; -1 because sketch_coverage_classification test went with the deleted SketchCoverage enum)
• cargo test -p query_engine_rust --lib -- engines::warm_tier — 13/13 pass

Grep verification (final)

• JoinSketch | QueryExpr::Subquery | QueryExpr::WindowFunc | SketchCoverage | ScalarExpr::{UnaryOp,InSubquery,Case,Cast,VectorBinaryOp} → zero matches.
• QueryExpr::Dedup | Dedup { → zero matches.
• Remaining ::Subquery\b hits (6) are Expr::Subquery(sq) from external promql_parser / sqlparser AST libraries, not our type.

Diff: 7 files, +66 / -323 (net -257 lines)

controller/src/intent_algebra/legacy_expr.rs   | 177 +-------------------------
controller/src/intent_algebra/legacy_lower.rs  |  20 +--
controller/src/optimizer/engine.rs             |  51 +++-----
controller/src/physical/allocator.rs           |  77 ++---------
controller/src/physical/planner.rs             |  29 ++++-
controller/src/physical/stage_split.rs         |  23 +---
controller/src/query_parser/mod.rs             |  12 +-

Next in the chain

Batch 2 — A-lifts (next PR): Lift Filter, Project, Partition, Distinct, Merge, Join, SetOp, Sort, Limit, BinaryOp into canonical intent_algebra::query_expr::QueryExpr + a minimal typed Predicate (Column/Literal/BinaryOp/IsNull) for Filter.pred. Then Batch 3 sub-PRs cover the C-reshapes (Source→Scan, WindowedAgg un-fusion, SketchAgg→Aggregate@L3, TopK split, AggFunc→AggIntent, PromQL parser-side histogram_quantile→Quantile substitution). Final PR retires legacy_expr.rs + legacy_lower.rs entirely.

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