feat: comprehensive codebase improvements for production readiness

[alexekdahl]

This major refactor enhances marksman.nvim's robustness, performance, and maintainability while preserving full backward compatibility. The plugin now provides enterprise-grade error handling, memory management, and user experience improvements.

Key Improvements

Robustness & Error Handling

• Comprehensive error handling with graceful fallbacks
• Automatic backup/restore for corrupted storage files
• Atomic file operations with validation
• Enhanced input validation with clear error messages

Performance & Memory

• Debounced save operations (configurable 100-5000ms)
• Lazy module loading with cleanup functions
• Project root caching with smart expiration
• Memory usage monitoring via get_memory_usage()

Code Quality & Documentation

• JSDoc-style documentation with @param/@return annotations
• Modular design with clear separation of concerns
• Comprehensive CONTRIBUTING.md for community development
• Linting configuration with .luacheckrc

API Improvements

• Standardized return values: {success, message, data}
• Configuration schema validation with constraints
• Enhanced mark validation and sanitization
• Expanded language support for smart naming

User Experience

• Better error feedback and notifications
• Enhanced UI with dynamic sizing and error handling
• Improved search functionality across multiple fields
• Statistics and diagnostics commands

New Configuration Options

• debounce_ms: Configurable save debouncing (500ms default)
• Enhanced validation for max_marks (1-1000 range)
• Backward-compatible configuration schema

Breaking Changes

None - all changes maintain full backward compatibility.

The codebase is now community-ready with comprehensive documentation, robust error handling, and optimized performance while maintaining the focused, single-purpose design philosophy.