feat(audio): Implement real-time spectrogram synthesizer

Adds a multi-voice, real-time audio synthesis engine that generates sound from spectrogram data using an Inverse Discrete Cosine Transform (IDCT).

Key features:
- A thread-safe, double-buffered system for dynamically updating spectrograms in real-time without interrupting audio playback.
- Core DSP components: FDCT, IDCT, and Hamming window functions.
- A simple sequencer in the main loop to demonstrate scripted audio events and dynamic updates.
- Unit tests for the new synth engine and Hamming window, integrated with CTest.
- A  file documenting the build process, features, and how to run tests.

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+# How To
+
+This document describes the common commands for building and testing the project.
+
+## Features
+
+*   **Real-time Audio Synthesis**: The demo features a multi-voice synthesizer that generates audio in real-time from spectrograms.
+*   **Dynamic Sound Updates**: Spectrograms can be updated dynamically and safely during runtime for evolving soundscapes.
+
+## Building
+
+### Debug Build
+
+```bash
+cmake -S . -B build
+cmake --build build
+```
+
+### Size-Optimized Build
+
+```bash
+cmake -S . -B build -DDEMO_SIZE_OPT=ON
+cmake --build build
+```
+
+## Testing
+
+To build and run the tests, you need to enable the `DEMO_BUILD_TESTS` option in CMake.
+
+Available test suites:
+*   `HammingWindowTest`: Verifies the properties of the Hamming window function.
+*   `SynthEngineTest`: Verifies the core functionality of the audio synthesizer.
+
+```bash
+cmake -S . -B build -DDEMO_BUILD_TESTS=ON
+cmake --build build
+cd build
+ctest
+cd ..
+```