docs: project-wide testing strategy

[priyanshujain]

Summary

• Replaces docs/spectest-design.md with docs/testing-strategy.md covering the full testing approach
• Each test level is defined by its perspective and the question it answers, not by vague labels
• Five perspectives: author (unit), consumer (contract), protocol (conformance), boundary (integration), user (spec)
• Includes concrete examples from the codebase, assertion strategies, naming conventions, and guidance on when to write which type of test

Test plan

• Read the document and verify it accurately reflects the project's testing philosophy