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.

.gitignore

@@ -7,8 +7,16 @@ __pycache__/
 *.so
 *.egg-info/
 dist/
-/build/*
+# Build outputs (ignore artifacts, track scripts/configs)
+/build/work/
+/build/downloads/
+/build/out/
+/build/*.img
+/build/*.img.xz
+!/build/*.sh
+!/build/*.md
 !/build/.gitkeep
+!/build/first-boot/
 *.egg
 .eggs/
 venv/