fix: zero BIOS boot partition to prevent FAT-fs errors, add CPU microcode

- dd zero the 1MB BIOS boot partition before formatting to prevent
  kernel FAT-fs bread() errors during boot (sda1 had stale data)
- Add intel-microcode and amd64-microcode packages to suppress
  TSC_DEADLINE and similar CPU firmware bug warnings on boot

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

image-recipe/build-auto-installer-iso.sh

@@ -236,6 +236,8 @@ RUN apt-get update && apt-get install -y \
     firmware-realtek \
     firmware-iwlwifi \
     firmware-misc-nonfree \
+    intel-microcode \
+    amd64-microcode \
     && apt-get clean \
     && rm -rf /var/lib/apt/lists/*
 
@@ -1135,6 +1137,8 @@ fi
 
 # Format partitions
 echo "   [2/6] Formatting partitions..."
+# Zero out the BIOS boot partition to prevent FAT-fs read errors during boot
+dd if=/dev/zero of="$BIOS_PART" bs=1M count=1 2>/dev/null || true
 mkfs.vfat -F32 -n EFI "$EFI_PART"
 mkfs.ext4 -F -L archipelago "$ROOT_PART"