refactor(legacy_expr): step β — thread Schema through legacy planning stack

[zzylol]

Summary

Plumbing-only refactor. After Step α (PR #138) gave us canonical AggIntent inside legacy QueryExpr variants, the next blocker for variant-by-variant consumer migration (Step γ) is that consumers can't resolve ColumnRef::Named(\"host\") → ColumnId(2) without a Schema.

Step β threads Schema through every consumer-side walker via traversal-environment parameter (approach c — lightest touch; mirrors how canonical QueryExpr already passes schema downward).

What landed

New: controller/src/intent_algebra/column_resolution.rs (256 lines, 8 tests)

pub fn resolve_column_ref(col: &ColumnRef, schema: &Schema)
    -> Result<ColumnId, ResolveError>;
pub fn resolve_column_refs(cols: &[ColumnRef], schema: &Schema)
    -> Result<Vec<ColumnId>, ResolveError>;

pub fn infer_schema_for_root(expr: &QueryExpr) -> Schema;
pub fn infer_source_schema(metric_name: &str) -> Schema;

infer_source_schema synthesises the default time-series shape (ts: Timestamp, value: Float64) per the deferred SchemaCatalog decision. infer_schema_for_root walks to the outermost Source leaf via existing QueryExpr::source_name() and applies the default; falls back to Schema::default() for bare Ref(_) trees.

Threading applied to

• optimizer::engine::RewriteRule::try_rewrite — trait + 12 impls
• optimizer::engine::QueryOptimizer::{apply_all, recurse_children, apply_rules_at}
• physical::allocator::SketchAllocator::{alloc_node, alloc_sketch_agg}
• physical::planner::plan_node
• physical::stage_split::{walk, promql_from_qe}
• query_parser::mod::QeCollector::visit
• intent_algebra::legacy_lower::lower_to_sketch_algebra_with_schema (new sibling; old fn delegates after deriving schema)

Public entry points (SketchAllocator::allocate, plan, split_expr_by_stage, expr_to_promql, lower_to_sketch_algebra, qe_to_parsed_query, QueryOptimizer::optimize) keep existing signatures — each derives the root schema and threads it inward. Backwards compatibility intact.

Build + test

• cargo build --release -p controller clean
• cargo build --release -p query_engine_rust clean
• cargo test --release -p controller --lib — 674 passed (was 666 after Step α; +8 new column_resolution tests)
• cargo test --release -p controller --bin controller — 27 passed

7 TODOs flagged for Step γ

1. Aggregate { keys: Vec<String> } → Vec<ColumnId> migration — needs schema transformation as we descend into Aggregate's child
2. Distinct { cols } consumer in physical::stage_split currently logs ColumnIds without using them — BackendSubPlan needs a distinct_cols: Vec<ColumnId> field
3. Project { cols } reshapes columns canonically; legacy passes parent schema through verbatim
4. LetBinding / Ref need a BindingScope analogue plumbed alongside Schema
5. Label sets (*labels) aren't representable in canonical Schema today — needs a DataType::LabelSet or parallel side-channel before full Source → Scan migration
6. CostModel::estimate(&self, expr: &QueryExpr) doesn't take Schema (inspects only outermost node). Future cost estimators may need the signature extended
7. optimizer::engine::HydraConversion (disabled in Step α) could now consult the schema to emit canonical Aggregate { by, aggs: [inner] } instead of inlining into SketchAgg.op

Diff: 8 files, +579 / -151

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