Build: Add USB boot + NVMe HAT support

- --target flag in build.sh (sd|usb|nvme)
- NVMe kernel driver enabled for --target nvme
- EEPROM boot-order config in first-boot wizard
- USB/NVMe flashing docs in build/README.md
- Hardware compatibility + user manual docs from P7

build/README.md

@@ -1,7 +1,8 @@
-# RPi Audio Mixer — SD Card Image Builder
+# RPi Audio Mixer — Disk Image Builder (SD / USB / NVMe)
 
 This directory contains the automated build system for creating a ready-to-flash
-Raspberry Pi 4B SD card image with the full audio mixer stack.
+Raspberry Pi 4B disk image with the full audio mixer stack. The same image works
+for SD cards, USB SSDs, and NVMe HATs — just choose your target.
 
 ## Quick Start
 
@@ -27,6 +28,8 @@ ls -lh build/out/rpi-audio-mixer-*.img.xz
 | `./build/build.sh --skip-kernel` | Quick build, stock kernel | 10-15 min |
 | `./build/build.sh --kernel-only` | Only build RT kernel | 10-15 min |
 | `./build/build.sh --clean` | Remove all build artifacts | < 1 min |
+| `./build/build.sh --target usb` | Build for USB SSD boot | same as default |
+| `./build/build.sh --target nvme` | Build for NVMe HAT (NVMe driver enabled) | +1 min |
 
 ## Flashing to SD Card
 
@@ -48,6 +51,42 @@ sync
 sudo eject /dev/sdX
 ```
 
+## Flashing to USB SSD / NVMe
+
+The **same image** works for USB SSD and NVMe HAT — it's just flashed to a different device.
+
+### USB SSD
+
+```bash
+# 1. Find your USB SSD device
+lsblk  # look for /dev/sda, /dev/sdb, etc.
+
+# 2. Flash the image
+sudo dd if=build/out/rpi-audio-mixer-*.img of=/dev/sda bs=4M status=progress conv=fsync
+
+# 3. On first boot, the wizard can configure EEPROM for USB boot
+#    Or do it manually:
+#    sudo rpi-eeprom-config --edit  →  BOOT_ORDER=0xf41
+```
+
+### NVMe HAT
+
+Requires an NVMe HAT (official RPi, Pimoroni, or Geekworm) connected via PCIe.
+
+```bash
+# 1. Flash to NVMe SSD (connect via USB adapter or directly on Pi)
+sudo dd if=build/out/rpi-audio-mixer-*.img of=/dev/nvme0n1 bs=4M status=progress conv=fsync
+
+# 2. Configure EEPROM for NVMe boot:
+#    sudo rpi-eeprom-config --edit  →  BOOT_ORDER=0xf614
+
+# 3. Boot order: NVMe → USB → SD → Restart
+```
+
+> **⚠️ USB performance note:** During audio operation, minimize writes to USB SSDs
+> as they share the USB bus with audio interfaces. NVMe HAT is recommended for
+> production use — it uses dedicated PCIe lanes with no USB bus contention.
+
 ### From macOS
 
 ```bash