v2.7.0 - Stream I/O robustness and code quality enhancement

🔧 Stream I/O Robustness:

• Comprehensive exception handling across all distribution stream input operators
• Consistent try-catch blocks with std::stod() parsing and robust error recovery
• Stream failbit setting on parse failure instead of throwing exceptions
• Standardized variable naming conventions and numeric boundary handling

📋 Gold Standard Checklist:

• Established comprehensive Gold Standard Checklist for distribution quality
• Documented consistent patterns for exception handling and error recovery
• All distributions meet robustness and maintainability standards
• 100% test pass rate with enhanced reliability

📚 Documentation & Organization:

• Moved CROSS_PLATFORM.md to docs/ directory
• Added cross-platform build guide references
• Updated Contributing section with style guide and checklist references
• Added clang-tidy enforcement for automated code quality

🏗️ Architecture & Build System:

• Reorganized test suite into logical subdirectories
• Enhanced CMake integration with cross-platform support
• Modern C++17 matrix/vector architecture with SIMD optimization
• .clang-tidy configuration for code quality enforcement

This release establishes solid foundations for consistent code quality and
robust stream I/O operations across the entire libhmm codebase.