Add inst.ks.sendmac to the iPXE boot script

Cobbler's per-MAC gPXE script passed inst.ks.sendmac, making anaconda send
its interface MACs as X-RHN-Provisioning-MAC-N headers when fetching the
kickstart. bootapi's iPXE script omitted it, so the boot script was not a
full drop-in for Cobbler's gpxe output.

- Emit inst.ks.sendmac immediately before inst.ks= in the boot iPXE template,
  matching Cobbler ordering.
- Assert the arg is present in the rendered catalog iPXE test.

Claude-Session: https://claude.ai/code/session_01JUoARVdmhxKQHyyyp1pxeT

internal/render/render_test.go

@@ -136,7 +136,7 @@ func TestRenderIPXECatalog(t *testing.T) {
 	mustContain(t, s, "kernel "+artifactBase+"/almalinux/9/BaseOS/x86_64/os/images/pxeboot/vmlinuz")
 	mustContain(t, s, "initrd "+artifactBase+"/almalinux/9/BaseOS/x86_64/os/images/pxeboot/initrd.img")
 	mustContain(t, s, "inst.repo="+artifactBase+"/almalinux/9/BaseOS/x86_64/os")
-	mustContain(t, s, "inst.ks=http://bootapi.example.net/ks/web01")
+	mustContain(t, s, "inst.ks.sendmac inst.ks=http://bootapi.example.net/ks/web01")
 	mustContain(t, s, "inst.text") // catalog kernel arg
 	mustContain(t, s, "net.ifnames=0")
 }