docs: Document specplay tool and add future enhancement roadmapHEADmain

- Created tools/specplay_README.md with comprehensive documentation
- Added Task #64 to TODO.md for future specplay enhancements
- Updated HOWTO.md with specplay usage examples and use cases
- Outlined 5 priority levels of potential features (20+ ideas)

Key enhancements planned:
- Priority 1: Spectral visualization, waveform display, frequency analysis
- Priority 2: Diff mode, batch analysis, CSV reports
- Priority 3: WAV export, normalization
- Priority 4: Advanced spectral analysis (harmonics, onsets)
- Priority 5: Interactive mode (seek, loop, volume control)

The tool is production-ready and actively used for debugging.

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