feat: Unified Query AST + Multi-Language Execution Engine

[muk2]

Summary

Implements the foundational architecture from RFC #7 — the Unified Query AST and Multi-Language Execution Engine for pgrsql.

This PR introduces a complete src/ast/ module with the following layers:

Core AST Types (types.rs)

• Query enum — top-level representation for SELECT, INSERT, UPDATE, DELETE, CTEs, and raw SQL passthrough
• SelectQuery — full SELECT support including DISTINCT, projections, FROM, JOINs, WHERE, GROUP BY, HAVING, WINDOW, ORDER BY, LIMIT/OFFSET, and set operations (UNION/INTERSECT/EXCEPT)
• Expression enum — recursive expression tree supporting columns, literals, binary/unary ops, function calls, aggregates, window functions, CASE, subqueries, EXISTS, IN, BETWEEN, IS NULL, CAST, arrays, JSON access, type casts, and parameters
• Join, TableRef, WindowSpec, CTE — complete relational algebra primitives

SQL Parser (parser.rs)

• Translates SQL text → unified AST using sqlparser crate with PostgreSQL dialect
• Supports all major SQL constructs: CTEs (including recursive), window functions with PARTITION BY, subqueries, set operations, LATERAL joins, JSON operators, aggregate functions with DISTINCT/FILTER
• Decoupled from sqlparser's AST — our representation can evolve independently

SQL Compiler (compiler.rs)

• Compiles AST → PostgreSQL SQL string
• Enables round-trip: SQL → AST → SQL (verified by tests)
• Foundation for DSL-to-SQL compilation paths

Language Adapter Layer (adapter.rs)

• QueryLanguageAdapter trait — for parsing raw query languages (SQL dialects)
• DSLAdapter trait — for Domain-Specific Languages that compile to our AST
• AdapterRegistry — manages multiple adapters with auto-detection
• Built-in PostgresAdapter registered by default

Optimization Infrastructure (optimizer.rs)

• OptimizationPass trait — pure transformation passes over the AST
• Optimizer — composable pipeline of ordered passes
• analyze_query() — structural introspection returning QueryAnalysis (detects joins, CTEs, aggregation, window functions, subqueries, set operations, JSON ops)
• Built-in RemoveRedundantNesting pass

Plugin Architecture (plugin.rs)

• QueryPlugin trait — register capabilities at startup
• PluginRegistry — central registry for adapters, DSLs, and optimization passes
• Clean extension point for future language integrations (Python DSL, OCaml, Lean, WASM)

Architecture

Input (SQL / DSL)
      ↓
Language Adapter Layer  (adapter.rs)
      ↓
Unified Query AST       (types.rs)
      ↓
Analysis / Optimization (optimizer.rs)
      ↓
SQL Compiler            (compiler.rs)
      ↓
Execution Engine        (existing db/ module)

Test plan

• 65 unit tests covering all modules
• Parser tests: simple SELECT, WHERE, JOIN, GROUP BY, CTE, recursive CTE, window functions, INSERT, UPDATE, DELETE, subqueries, UNION, ORDER BY, BETWEEN, CASE, IS NULL, CAST, DISTINCT, multiple statements, invalid SQL
• Compiler round-trip tests: parse SQL → compile → verify re-parseable
• Optimizer tests: empty pipeline, default passes, nesting removal, query analysis
• Adapter tests: PostgreSQL adapter detection, registry auto-dispatch
• Plugin tests: loading, registration, capability discovery
• All clippy warnings resolved
• cargo fmt applied

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