refactor(controller): retire algebra/+planner/+config/+query_language/+analyzer.rs (Steps 2b/c/d/e/f)

[zzylol]

Summary

Per docs/design.md §5 target layout. Restructure controller/src/ from the historic mixed layout (algebra/ + planner/ + config/ as ill-defined catch-alls, plus stage_split.rs + analyzer.rs scattered top-level files) into the design.md modular split.

This is Step 2b/c/d/e/f from the architectural cleanup chain that started with #129 (Step 2a — capability consolidation).

Old → new path mapping

Old	New
algebra/expr.rs	intent_algebra/legacy_expr.rs (kept; 91 consumers — unification follow-up)
algebra/lower.rs	intent_algebra/legacy_lower.rs
algebra/{directory,physical,allocator,plan}.rs	physical/{sketch_catalog,planner,allocator,plan}.rs
algebra/optimizer.rs	optimizer/engine.rs
planner/{cost_model,delta,online,pareto,tco,wire_cost}.rs	optimizer/cost/{mod,delta,online,pareto,tco,wire}.rs
planner/rules.rs	optimizer/rules/mod.rs
planner/baseline_planner.rs	optimizer/baseline.rs
planner/stage_split.rs	physical/stage_split.rs
analyzer.rs	pipeline.rs
stage_split/	physical/colored_dag/ (colored_dag.rs → dag.rs)
query_language/	query_parser/language/
config/workloads.rs	workload.rs
config/{agent,backend,asapquery_backend,precompute,stage_config,stage_config_otap,stage_config_telegraf}.rs	emit/{agent,backend,asapquery_backend,precompute,stage_config,otap,telegraf}.rs
(new)	physical/topology.rs, emit/trait_def.rs, optimizer/trait_def.rs, deployment_model.rs

Pragmatic concessions (4 documented TODOs in §16.2)

1. algebra/expr.rs → intent_algebra/legacy_expr.rs — the original brief assumed only one stray consumer existed of algebra::expr::AggIntent. Reality: ~91 sites across query_parser, language_logical_plan, planner, algebra, config use the legacy QueryExpr/AggIntent/WindowSpec IR. Treating these as "duplicates to delete" would have required rewriting every consumer. The file is moved with a doc-comment marking it as legacy IR pending unification with canonical intent_algebra::{agg_intent,query_expr,schema}. Same for algebra/lower.rs → intent_algebra/legacy_lower.rs.
2. config/stage_config.rs (3,020 lines) moved whole as emit/stage_config.rs rather than split into emit/{opamp,streaming_config,inference_config}.rs. The monolith mixes OTel-collector YAML, ASAPQuery-backend JSON, storage-routing JSON, and shared internals; a clean split needs ownership reorganisation, not file renames.
3. Back-compat module aliases in lib.rs (pub use intent_algebra::legacy_expr as algebra; etc.) — controller/src/main.rs references crate::stage_split::*, crate::algebra::*, etc. (bin crate). Keeping shims lets main.rs continue compiling without per-line edits.
4. hll_accuracy() / countmin_accuracy() moved from legacy_expr.rs → sketch_algebra/capability.rs (their structural home). legacy_expr.rs retains thin pub use re-exports.

docs/design.md updates

• §3 layer table — rows rewritten with new paths + *Refactor 2026-05 absorbed …* italicised notes.
• §5 Target repo layout — blockquote: multi-crate split deferred; lookup table mapping crates/core/* → single-crate paths.
• §6 Core crate details — blockquote: 16-row lookup table.
• §16 NEW — ADR section. §16.1 documents PR refactor: consolidate Capability into sketch_algebra; warm_tier_analysis becomes facade (Step 2a) #129's capability consolidation; §16.2 documents this PR's retirement with the full mapping table and four TODOs.

Build + test

• cargo build --release -p controller — clean (4 pre-existing warnings)
• cargo build --release -p query_engine_rust — clean (3 pre-existing warnings)
• cargo test -p controller --lib — 633/633 pass (unchanged from refactor: consolidate Capability into sketch_algebra; warm_tier_analysis becomes facade (Step 2a) #129)
• cargo test -p query_engine_rust --lib -- engines::warm_tier — 13/13 pass

Diff: 55 files changed, +568 / -309

(File renames + import-path rewires. Net additions are mostly the new placeholder trait files + the design.md §16 ADR.)

Follow-up PRs queued

1. Migrate consumers of intent_algebra::legacy_expr::* onto canonical types; delete the legacy_*.rs files.
2. Split emit/stage_config.rs (3,020 lines) per design.md §5.
3. Implement the placeholder traits (OptimizerRule, PlanEmitter, DeploymentModelRegistry).
4. Migrate controller/src/main.rs off the back-compat module aliases; remove the shims.
5. sketch_algebra/ touch-up (the user's open request): drop AggIntent::HistogramQuantile; map Min/Max → QuantileApprox; consolidate SketchCapability vs SketchCapabilities via the "rename schema.rs::SketchCapabilities → SketchStateMetadata" approach; rename params.rs → sketch_params.rs; add FrequencyEstimate(SketchKindHandle) capability variant + CountSketchWithHeap handle. PromQL/MetricsQL surface: sum by (item) (rate(m[r])) → FrequencyEstimate; topk(k, …) → FrequencyTopk.

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