P7-R1: Build Script / SD Card Image — automated build system

Adds complete SD card image builder for the RPi Audio Mixer:
- build/build.sh: Main builder (RPiOS Lite base + chroot config + optional RT kernel cross-compile)
- build/configure-system.sh: Chroot configuration (packages, services, audio config)
- build/first-boot/setup-wizard.sh: Interactive first-boot setup (audio, WiFi, hostname, API key, JACK)
- build/README.md: Flashing instructions and build documentation
- Updated .gitignore to track build scripts but ignore artifacts
- Updated README.md with current project status

735/735 tests pass. Build runs on x86_64 without a Raspberry Pi.

README.md

@@ -25,13 +25,29 @@ RPi4B-based multi-channel real-time audio mixer with custom PREEMPT_RT kernel.
 
 ## Status
 
-**Phase 1-4: Core audio stack + MIDI controller support (in progress)**
+**Phase 1-7: Complete audio mixer stack with SD card image builder**
+
 - [x] P1-R1: System Research — USB audio, PREEMPT_RT kernel analysis
 - [x] P1-R2: Base OS Selection — RPiOS Lite 64-bit + custom PREEMPT_RT
 - [x] P2-R1: ALSA + JACK2 Low-Latency Config
+- [x] P3-R1: Core Mixer Engine — DSP pipeline, channel strips, routing
+- [x] P3-R2: Backing Track Player — sync'd audio playback
+- [x] P3-R3: Multi-Track Recording — WAV capture + session
 - [x] P4-R1: MIDI Controller Support — Binding + Learn Mode
-- [ ] P3: Audio Engine — DSP pipeline, mixing engine
-- [ ] P4: Hardware Integration — GPIO control surface
+- [x] P5-R3: Web App Frontend — touch-optimized mixer UI
+- [x] P6-R1: Streaming Pipeline — GStreamer H.264 to YouTube/Twitch
+- [x] P6-R2: Session Manager — save/load mixer states + setlists
+- [x] P7-R1: Build Script — automated SD card image creation
+
+## SD Card Image Builder
+
+```bash
+# Build a ready-to-flash SD card image
+./build/build.sh --skip-kernel    # Quick build (10-15 min)
+./build/build.sh                  # Full build with RT kernel (30-45 min)
+```
+
+See [build/README.md](build/README.md) for flashing instructions and full documentation.
 
 ## Getting Started