code-contracts-live-query.md

@tanstack/db	patch

Add code contracts for live query D2 multiplicity invariant.

Introduces a contract-based approach to ensure correctness in the live query system, inspired by Cheng Huang's article on AI-assisted development. Contracts verify runtime invariants during development/testing and can be disabled in production.

Key additions:

• contracts.ts with precondition(), postcondition(), and invariant() utilities
• D2 multiplicity contracts in CollectionSubscriber.sendChangesToPipeline() ensuring no duplicate keys are sent to the incremental view maintenance pipeline
• 16 contract verification tests covering multiplicity, tracking, and consistency
• 9 property-based tests using fast-check to explore edge cases with random operation sequences

Contracts are automatically disabled when NODE_ENV=production for zero runtime overhead in production builds.