feat(controller): Phase D — query_language L1 trait + language_logical_plan L2 + PromQL backend

[zzylol]

Summary

Phase D of the controller-design alignment per controller/docs/design.md §6: introduces L1 (query_language) + L2 (language_logical_plan) and wraps the existing PromQL parser behind a multi-language Language trait. PromQL is the only active backend; Sql, DataFusion, ElasticDsl are stubbed with ParseError::Unimplemented so the type system is uniform while the DC build ships PromQL only.

• L1 (controller/src/query_language/): Language trait + LanguageAst sum + per-language sub-modules. PromQL backend is a thin adapter over query_parser::{parse_query, parse_query_expr} — no parsing logic duplicated.
• L2 (controller/src/language_logical_plan/): LanguageLogicalPlan (one variant per language) + lower_to_logical_plan(LanguageAst) -> LanguageLogicalPlan pass. PromQL's L2 IS the existing QueryExpr tree (rebadged + paired with a flat LanguageLogicalPlanSummary projection).
• Legacy query_parser/ is read-only — its public surface (parse_query, parse_query_expr, ParsedQuery) stays unchanged for back-compat with analyzer::Analyzer.

Module layout

controller/src/query_language/
  ├── language.rs          — Language trait + ParseError
  ├── language_ast.rs      — LanguageAst sum
  ├── promql/{mod,ast}.rs  — active PromQL backend (wraps query_parser)
  ├── sql/mod.rs           — stub (Unimplemented)
  ├── datafusion/mod.rs    — stub (Unimplemented)
  ├── elastic_dsl/mod.rs   — stub (Unimplemented)
  └── tests.rs             — 8 unit tests

controller/src/language_logical_plan/
  ├── lower.rs             — lower_to_logical_plan + LoweringError
  ├── plan.rs              — LanguageLogicalPlan + LanguageLogicalPlanSummary
  ├── mod.rs               — re-exports
  └── tests.rs             — 6 unit tests

Coordination with Phase B

controller/src/main.rs add: mod query_language; mod language_logical_plan; — may conflict with Phase B's mod intent_algebra;. Trivial 3-way merge. The crate has no lib.rs (bin-only), so module declarations live in main.rs instead — orchestrator should treat both phases' edits the same way regardless.

Pre-existing test breakage (NOT introduced here)

origin/main at be44752 (post-#273) has 9 pre-existing E0063 errors in test code in controller/src/config/{agent,asapquery_backend,precompute}.rs and controller/src/main.rs::api_tests — all reading missing field 'data_sink' in initializer of types::AgentCollectorConfig. #273 added data_sink to AgentCollectorConfig but missed those test sites. Per Phase D constraints I cannot touch types.rs, config/, or main.rs::api_tests, so I left them alone. Consequence:

• cargo build --release -p controller — clean (my new modules add zero warnings).
• cargo check --release --tests for query_language / language_logical_plan — clean (my new test code compiles).
• cargo test --release -p controller — fails to build due to pre-existing feat(controller): additive QuerySpec types from design.md (QueryShape/DataShape/AccuracyTarget/QueryLanguage) #273 oversight, NOT due to anything in this PR.
• cargo clippy --release -p controller --all-targets -- -D warnings — fails on pre-existing KLL/HLL/etc. naming-acronym warnings in types.rs; zero clippy hits in this PR's new modules (cargo clippy --release filtered to query_language|language_logical_plan returns nothing).

Recommend a follow-up trivial fix-test PR (or amend #273) to populate data_sink in those constructors.

Test plan

• After the data_sink follow-up lands: cargo test --release -p controller query_language — all 8 new tests green.
• After the data_sink follow-up lands: cargo test --release -p controller language_logical_plan — all 6 new tests green.
• After the data_sink follow-up lands: cargo test --release -p controller query_parser — existing tests not regressed (this PR does not modify query_parser/).
• cargo build --release -p controller clean.
• No new clippy warnings on query_language/ or language_logical_plan/.
• Submodule pointers unchanged.

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