feat: improve playback speed quality with Hann windowing and fraction…

…al tracking

- Replaced linear cross-fading with a Hann (Raised Cosine) window to eliminate "metallic" artifacts.
- Implemented fractional input position tracking to ensure smooth, high-precision speed control at all values.
- Optimized WSOLA template matching logic to improve phase coherence.
- Increased window and overlap sizes (60ms/20ms) for better low-frequency response.
- Added a technical study on audio optimization in Study.md.

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