Add Lean 4 formal verification layer for query optimizer

[muk2]

Summary

Implements Phase 1 of the formal verification initiative (RFC #27) by integrating Lean 4 as a proof layer for pgrsql's query optimizer. This transforms optimizer correctness from a testing problem into a mathematical guarantee.

• Lean 4 project (lean/) with Lake build system, pinned to Lean 4.28.0
• Formal relational algebra: Types for Values (with NULL), Tuples, Relations, and three-valued logic (TVL)
• Relational algebra operators: Selection (σ), Projection (π), Join (⨝), Rename (ρ), Union, Intersection, Difference, Cross Product
• 15 machine-checked theorems proving rewrite rule correctness, including:
  • Filter merge: σ_c(σ_d(R)) = σ_(c∧d)(R)
  • Selection commutativity & idempotence
  • Predicate pushdown through UNION
  • De Morgan's laws for three-valued logic
  • Empty relation elimination
  • Join decomposition into selection over cross product
• NULL semantics formalization: Full 3VL truth tables verified, NULL propagation properties, absorption laws, associativity, distributivity
• Executable examples validated at compile time with native_decide
• CI workflow (.github/workflows/lean.yml) that builds and verifies all proofs on push/PR
• Comprehensive documentation (LEAN.md) covering architecture, how to build/test, contribution guide, and future roadmap

What this proves

Every theorem in Verification/Theorems.lean and Verification/NullSemantics.lean is machine-checked by Lean's type system. If the build succeeds, the proofs are valid for all possible inputs — not just test cases.

What's next (Phase 2+)

• Complete the project_idempotent structural proof (currently sorry)
• Prove join associativity, CTE inlining equivalence
• Formalize aggregation operators
• Bridge Lean specifications with Rust optimizer implementation
• Full outer join and advanced NULL semantics

Closes #27

Test plan

• lake build succeeds with all proofs verified (only expected sorry warning for Phase 2 target)
• Verification.Examples compile-time tests pass via native_decide
• CI workflow triggers on lean/** path changes
• No changes to existing Rust code — zero risk of regression
• LEAN.md documents setup, build, test, and contribution workflow

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