P1-R3: Initial project infrastructure

- Project directory structure: docs/, src/, build/, recipes/, scripts/, wiki/
- README.md with project overview and status
- .editorconfig for consistent code style
- .gitignore for Python, C/C++, kernel, and build artifacts
- docs/research/: system research report + base OS decision
- docs/audio-stack-config.md: audio stack configuration notes
- config/: JACK, ALSA, CPU performance configs
- Preserved research from P1-R1 and P1-R2 tasks

README.md

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+# Raspberry Pi Real-Time Audio Mixer
+
+RPi4B-based multi-channel real-time audio mixer with custom PREEMPT_RT kernel.
+
+## Architecture
+
+- **Platform:** Raspberry Pi 4 Model B (8GB RAM)
+- **OS:** Raspberry Pi OS Lite (64-bit, Bookworm) + custom PREEMPT_RT kernel
+- **Audio:** USB audio interface with multi-channel I/O
+- **Goal:** Low-latency (<10ms round-trip) multi-channel audio mixing
+
+## Project Structure
+
+```
+├── docs/           Research, architecture decisions, manuals
+│   ├── research/   Technical research reports
+│   ├── decisions/  Architecture Decision Records (ADRs)
+│   └── manuals/    Hardware/software manuals and references
+├── src/            Source code (audio engine, DSP, UI)
+├── build/          Build scripts and cross-compilation tooling
+├── recipes/        Buildroot/Yocto recipes (if applicable)
+├── scripts/        Utility and deployment scripts
+└── wiki/           Local wiki pages
+```
+
+## Status
+
+**Phase 1: Research & Planning (in progress)**
+- [x] P1-R1: System Research — USB audio, PREEMPT_RT kernel analysis
+- [x] P1-R2: Base OS Selection — RPiOS Lite 64-bit + custom PREEMPT_RT
+- [ ] P1-R3: Project Infrastructure (current)
+- [ ] P2: Kernel Build — custom PREEMPT_RT kernel compilation
+- [ ] P3: Audio Engine — JACK/PipeWire integration, DSP pipeline
+- [ ] P4: Hardware Integration — USB interface, GPIO control surface
+
+## Getting Started
+
+*Documentation coming in Phase 2+*
+
+## License
+
+TBD