docs: Document specplay tool and add future enhancement roadmap

- 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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+# specplay - Audio Analysis & Playback Tool
+
+Standalone diagnostic tool for analyzing and playing .spec spectrogram files and .wav audio files.
+
+## Usage
+
+```bash
+./build/specplay <file.spec|file.wav>
+```
+
+## Features
+
+### Current (v1.0)
+- ✅ Plays .spec files via IDCT synthesis (matches demo playback exactly)
+- ✅ Plays .wav files for comparison
+- ✅ Reports PCM statistics:
+  - **Peak level**: Maximum absolute sample value (detects clipping if > 1.0)
+  - **RMS level**: Root-mean-square energy (loudness measure)
+- ✅ Real-time clipping detection during playback with sample position
+- ✅ Validates .spec file format (magic bytes, version, DCT size)
+
+### Example Output
+```
+Loading .spec: version=1, dct_size=512, frames=68
+PCM stats: Peak=0.403, RMS=0.058
+Playing 1.09 seconds... Press Ctrl+C to stop.
+Playback complete.
+```
+
+## Use Cases
+
+1. **Debugging Audio Issues**
+   - Quickly identify which samples have clipping (Peak > 1.0)
+   - Compare .wav source vs .spec output to detect analysis artifacts
+   - Verify spectrogram regeneration after DCT changes
+
+2. **Quality Assurance**
+   - Batch test all .spec files for clipping before committing
+   - Measure loudness consistency across samples (RMS levels)
+   - Validate spectrograms match expected characteristics
+
+3. **Development Workflow**
+   - Test individual samples without running full demo
+   - A/B compare different spectrogram generation parameters
+   - Verify procedural note generation output
+
+## Technical Details
+
+- **Sample Rate**: 32kHz (matches demo audio engine)
+- **Format**: Mono (duplicated to stereo for playback)
+- **IDCT**: Uses same `idct_512()` function as demo (bit-exact)
+- **Clamping**: Clipping detected but samples clamped to [-1, 1] for playback
+
+## Future Enhancement Ideas
+
+### Priority 1: Analysis Features
+- [ ] **Spectral visualization**: ASCII art frequency plot
+- [ ] **Waveform display**: Time-domain amplitude graph
+- [ ] **Frequency analysis**: Dominant frequency, spectral centroid
+- [ ] **Dynamic range**: Measure headroom, suggest normalization
+- [ ] **Duration info**: Show seconds, samples, frames
+
+### Priority 2: Comparison Tools
+- [ ] **Diff mode**: `specplay --compare file.wav file.spec`
+  - Side-by-side PCM stats
+  - RMS error, peak difference
+  - Correlation coefficient
+- [ ] **Batch mode**: `specplay --batch assets/final/*.spec`
+  - Generate CSV report of all stats
+  - Sort by peak level (find clipping candidates)
+  - Find outliers (unusually loud/quiet samples)
+
+### Priority 3: Export Features
+- [ ] **WAV export**: `specplay file.spec --export output.wav`
+  - Convert .spec → .wav for external analysis
+  - Useful for importing into audio editors
+- [ ] **Normalization**: `specplay file.spec --normalize --export normalized.wav`
+  - Auto-scale to target peak/RMS
+  - Preserve transients
+
+### Priority 4: Advanced Analysis
+- [ ] **Spectral envelope**: Extract formants, resonances
+- [ ] **Harmonic analysis**: Detect fundamental frequency, harmonics
+- [ ] **Onset detection**: Find transient attacks (kick/snare hits)
+- [ ] **Spectral flux**: Measure frequency content change over time
+
+### Priority 5: Interactive Mode
+- [ ] **Seek controls**: Play from specific time offset
+- [ ] **Loop mode**: Repeat playback indefinitely
+- [ ] **Volume control**: Adjust playback gain
+- [ ] **Real-time waveform**: Live visualization during playback
+
+## Integration with Build System
+
+Built automatically when `DEMO_BUILD_TOOLS=ON`:
+```bash
+cmake -S . -B build -DDEMO_BUILD_TOOLS=ON
+cmake --build build --target specplay
+```
+
+Or with all options:
+```bash
+cmake -S . -B build -DDEMO_ALL_OPTIONS=ON
+cmake --build build
+```
+
+## Code Architecture
+
+- **Minimal dependencies**: Only uses audio subsystem + miniaudio
+- **Self-contained**: No GPU, no platform layer (pure audio)
+- **Size**: ~250 lines (easy to extend)
+- **Format support**:
+  - `.spec` via SpecHeader parser
+  - `.wav` via miniaudio decoder (also handles .aif, .mp3)
+
+## Related Tools
+
+- **spectool**: Analyzes .wav → generates .spec (batch processing)
+- **specview**: ASCII visualization of .spec frequency content
+- **specplay**: This tool (playback + analysis)
+
+## Troubleshooting
+
+**"Failed to read SpecHeader"**
+- File is not a valid .spec file
+- Try with spectool to regenerate: `./build/spectool analyze input.wav output.spec`
+
+**"Peak exceeds 1.0! Will clip during playback."**
+- Spectrogram has too much energy
+- Likely needs regeneration after DCT changes
+- Or source .wav is already clipping
+
+**No audio output**
+- Check system audio device
+- Ensure 32kHz sample rate is supported
+- Try with a different .spec file (may be empty/silent)
+
+## Examples
+
+### Find all clipping samples
+```bash
+for f in assets/final/*.spec; do
+  ./build/specplay "$f" | grep "WARNING" && echo "$f"
+done
+```
+
+### Compare wav source to spec output
+```bash
+./build/specplay assets/originals/kick.wav
+./build/specplay assets/final/KICK.spec
+# Compare Peak/RMS values
+```
+
+### Quick loudness check
+```bash
+./build/specplay assets/final/KICK_606.spec | grep "RMS"
+# RMS > 0.3: loud, RMS < 0.1: quiet
+```
+
+---
+
+**Last Updated**: February 6, 2026
+**Author**: Claude (AI assistant)
+**Status**: Production-ready, actively used for debugging