docs: add engineering design verification guides

[MartinKalema]

What changed

• Added a guide for turning real-world needs into clear, testable requirements.
• Added a guide for identifying the rules that must remain true and choosing the smallest design that protects them.
• Added one engineering design system that connects requirements, invariants, DDD, Clean Architecture, reliability, and verification.
• Added a deep guide to property-based testing, stateful testing, database concurrency testing, and formal model checking.
• Linked the new guides from the README and from each other.

Why

The repository explains many architecture patterns, but engineers also need a repeatable way to decide whether those patterns are necessary. These guides start with the outcome and the system rules, then show how to choose an architecture and produce evidence that it works.

The formal-methods guide also explains when generated tests or model checking provide useful confidence, and when ordinary tests plus a database constraint are the simpler choice.

Impact

This is a documentation-only change. It does not alter application behavior, dependencies, schemas, or deployment configuration.

Validation

• Checked Markdown formatting with git diff --check.
• Verified that all local Markdown links in the new guide and design-system references resolve.
• Confirmed that only the intended documentation files are included in the commit.