Use sanhook to expose ISO as block device during kernel/initrd boot

The CPIO-embedded initrd approach failed (too large for iPXE memory).
Instead, use iPXE's sanhook to connect the ISO as a SAN block device
before booting with kernel/initrd. The installer finds the ISO on the
SAN device while we retain control over kernel parameters.

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

internal/pxe/ipxe.go

@@ -9,14 +9,17 @@ import (
 func BuildIPXEScript(publicURL string, img *model.Image, mac string) string {
 	kernelURL := fmt.Sprintf("%s/images/boot/%s", publicURL, img.KernelPath)
 	initrdURL := fmt.Sprintf("%s/images/boot/%s", publicURL, img.InitrdPath)
+	isoURL := fmt.Sprintf("%s/images/boot/%s", publicURL, img.ISOPath)
 
 	return fmt.Sprintf(`#!ipxe
 echo Provisioning: booting %s on ${mac}
+echo Connecting ISO via SAN...
+sanhook %s
 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)...
+kernel %s ramdisk_size=16777216 vga=791 video=vesafb:lfb:on rw quiet splash=verbose initrd=initrd
+echo Loading initrd...
 initrd --name initrd %s
 echo Booting...
 boot
-`, img.Name, kernelURL, initrdURL)
+`, img.Name, isoURL, kernelURL, initrdURL)
 }