Add ISO build tooling and serial console support

- Add bin/yino-iso with download, build, and clean subcommands for
  creating a preseeded Debian installer ISO
- Add --serial flag to bin/yino-vm boot for headless serial console
- Add offline installation ISO spec to FSD (section 6)
- Update README TODO to reflect ISO pipeline progress

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

bin/yino-vm

@@ -138,8 +138,13 @@ cmd_boot() {
     fi
 
     local iso_args=()
+    local serial=false
     while [[ $# -gt 0 ]]; do
         case "$1" in
+            --serial)
+                serial=true
+                shift
+                ;;
             --iso)
                 if [[ -z "${2:-}" ]]; then
                     echo "Error: --iso requires a path argument." >&2
@@ -154,14 +159,23 @@ cmd_boot() {
                 ;;
             *)
                 echo "Error: Unknown option: $1" >&2
-                echo "Usage: yino-vm boot [--iso PATH]" >&2
+                echo "Usage: yino-vm boot [--serial] [--iso PATH]" >&2
                 exit 1
                 ;;
         esac
     done
 
+    local display_args
+    if [[ "$serial" == true ]]; then
+        display_args=(-serial mon:stdio -nographic -display none)
+    else
+        display_args=(-display cocoa -vga virtio)
+    fi
+
     echo "Booting VM (accel=$accel, cpu=$cpu)..."
-    echo "SSH available at: ssh -p 2222 localhost"
+    if [[ "$serial" != true ]]; then
+        echo "SSH available at: ssh -p 2222 localhost"
+    fi
     echo ""
 
     qemu-system-x86_64 \
@@ -175,8 +189,7 @@ cmd_boot() {
         -drive "if=pflash,format=raw,file=$UEFI_VARS" \
         -device virtio-net-pci,netdev=net0 \
         -netdev user,id=net0,hostfwd=tcp::2222-:22 \
-        -display cocoa \
-        -vga virtio \
+        "${display_args[@]}" \
         "${iso_args[@]}"
 }
 
@@ -226,7 +239,7 @@ Usage: yino-vm <command> [options]
 
 Commands:
   create          Create VM disk image and UEFI vars
-  boot [--iso P]  Boot the VM (--iso to attach installer ISO)
+  boot [opts]     Boot the VM (--serial for console, --iso P for installer)
   delete          Delete VM disk image and UEFI vars
   status          Show VM status
   help            Show this help