Fix broken equal loudness filter and add comprehensive test suite

[Ranjuna120]

🎵 Fix Broken Equal Loudness Filter Implementation

📋 Summary

This PR fixes the broken equal loudness filter in the audio_filters module and significantly enhances the audio processing capabilities of the repository.

🔧 Changes Made

Fixed Core Issues

• Converted equal_loudness_filter.py.broken.txt to a working Python implementation
• Removed external dependency on yulewalker library
• Implemented NumPy-based Yule-Walker approximation for better compatibility

Added Comprehensive Testing

• Created audio_filters/tests/ directory with full test infrastructure
• Added 25+ comprehensive test cases covering all functionality
• Included edge cases, stability tests, and error handling validation
• Added test module initialization and structure

Enhanced Documentation & Examples

• Updated audio_filters/README.md with detailed usage examples
• Added comprehensive docstrings with mathematical references
• Created interactive demo script (demo_equal_loudness_filter.py)
• Updated module __init__.py for proper exports

Code Quality Improvements

• Added full type hints throughout the implementation
• Implemented proper error handling and input validation
• Added filter reset functionality and configuration info methods
• Followed project style guidelines and best practices

🎯 Technical Details

The equal loudness filter compensates for human auditory perception based on Robinson-Dadson equal loudness contours (1956). The implementation:

• Uses cascaded IIR filters (Yule-Walker approximation + Butterworth high-pass)
• Supports sample rates ≥ 44.1kHz
• Includes comprehensive numerical stability measures
• Provides educational value for psychoacoustic filtering concepts

📊 Impact

• ✅ Fixes broken functionality - Converts non-working .txt file to working implementation
• ✅ Adds educational value - Comprehensive examples and scientific references
• ✅ Improves code quality - Full type hints, tests, and documentation
• ✅ Zero breaking changes - Maintains backward compatibility
• ✅ Self-contained - No new external dependencies

🧪 Testing

# Quick test - all should pass
from audio_filters import EqualLoudnessFilter

filter = EqualLoudnessFilter(44100)
assert filter.process(0.0) == 0.0  # Silence test
assert isinstance(filter.process(0.5), float)  # Basic processing
filter.reset()  # State reset
info = filter.get_filter_info()  # Configuration info