Embed ISO in initrd for PXE boot with loop-mount wrapper

Instead of sanboot (which can't pass kernel params for automation),
switch back to kernel/initrd boot. The ISO is embedded in the initrd
as a CPIO append. A pxe-init wrapper script loop-mounts the ISO
before handing off to the original init, so the installer finds it
as a block device. Uses rdinit=/pxe-init kernel parameter.

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

internal/pxe/ipxe.go

@@ -7,11 +7,16 @@ import (
 )
 
 func BuildIPXEScript(publicURL string, img *model.Image, mac string) string {
-	isoURL := fmt.Sprintf("%s/images/boot/%s", publicURL, img.ISOPath)
+	kernelURL := fmt.Sprintf("%s/images/boot/%s", publicURL, img.KernelPath)
+	initrdURL := fmt.Sprintf("%s/images/boot/%s", publicURL, img.InitrdPath)
 
 	return fmt.Sprintf(`#!ipxe
 echo Provisioning: booting %s on ${mac}
-echo Booting ISO via HTTP SAN...
-sanboot %s
-`, img.Name, isoURL)
+echo Loading kernel...
+kernel %s ramdisk_size=16777216 vga=791 video=vesafb:lfb:on rw quiet splash=verbose rdinit=/pxe-init initrd=initrd
+echo Loading initrd (this may take a few minutes)...
+initrd --name initrd %s
+echo Booting...
+boot
+`, img.Name, kernelURL, initrdURL)
 }