Add English translation of chapter-06.md for website.en

[Copilot]

This PR provides a complete English translation of chapter-06.md ("Test-Driven Development and Testable Design") for the English version of the website.

Translation Details

The translated chapter includes comprehensive coverage of:

• Testable design principles and their impact on code quality
• Unit testing fundamentals following F.I.R.S.T principles (Fast, Independent, Repeatable, Self-Validating, Timely)
• Test-Driven Development (TDD) with Red-Green-Refactor cycle
• Test code quality improvement techniques and best practices
• Practical TDD exercises including a complete library management system implementation
• AI-era testing strategies for modern development workflows

Translation Quality

• Complete accuracy: All 1,711 lines translated from Japanese to American English
• Code preservation: All 50+ code examples in C#, Python, Java, and JavaScript preserved exactly with only comments translated
• Technical consistency: Maintained consistent terminology throughout (e.g., "testable," "mock," "stub," "dependency injection")
• Structure integrity: Preserved all markdown formatting, headers, tables, and links
• Image compatibility: All image references properly point to the website.en/Images/ directory

Validation Results

• ✅ Website functionality tested and confirmed working
• ✅ All image links accessible (chapter-06-banner.svg and diagrams)
• ✅ No inappropriate Japanese text remaining (only one intentional example in naming convention section)
• ✅ UTF-8 encoding without BOM
• ✅ JavaScript syntax validation passed

The English version is now available at website.en/MarkDown/chapter-06.md and integrates seamlessly with the existing English website structure.

Fixes #174.

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Pull Request Overview

This PR provides a complete English translation of Chapter 6 ("Test-Driven Development and Testable Design") for the English version of the website. The translation covers comprehensive TDD principles, testable design patterns, unit testing fundamentals with F.I.R.S.T principles, practical TDD exercises including a complete library management system, and modern AI-era testing strategies.

Key changes include:

• Complete 1,711-line translation from Japanese to American English with technical consistency
• Preservation of all 50+ code examples across multiple languages (C#, Python, Java, JavaScript) with translated comments only
• Integration with English website structure including proper image references to website.en/Images/