Add KVM provider pure builders: domain XML, volumes, seed ISO, metadata

Introduces internal/provider/kvm with the side-effect-free pieces of the
provider: providerID format/parse, the namespaced <metadata> element that
carries the GC contract, q35 domain XML over pool volumes, volume XML for
overlays and raw seeds, the NoCloud seed ISO built with kdomanski/iso9660,
and config defaults. All unit-tested.

Step 2 of docs/plans/2026-09-04-1404-kvm-provider.md.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

go.mod

@@ -5,6 +5,7 @@ go 1.26.0
 require (
 	cloud.google.com/go/compute v1.65.0
 	github.com/go-logr/logr v1.4.3
+	github.com/kdomanski/iso9660 v0.4.0
 	github.com/onsi/ginkgo/v2 v2.27.4
 	github.com/onsi/gomega v1.39.0
 	github.com/prometheus/client_golang v1.23.2
@@ -78,17 +79,17 @@ require (
 	go.uber.org/zap v1.27.1 // indirect
 	go.yaml.in/yaml/v2 v2.4.3 // indirect
 	go.yaml.in/yaml/v3 v3.0.4 // indirect
-	golang.org/x/crypto v0.54.0 // indirect
+	golang.org/x/crypto v0.56.0 // indirect
 	golang.org/x/exp v0.0.0-20251219203646-944ab1f22d93 // indirect
-	golang.org/x/mod v0.37.0 // indirect
+	golang.org/x/mod v0.38.0 // indirect
 	golang.org/x/net v0.57.0 // indirect
 	golang.org/x/oauth2 v0.36.0 // indirect
 	golang.org/x/sync v0.22.0 // indirect
 	golang.org/x/sys v0.47.0 // indirect
 	golang.org/x/term v0.45.0 // indirect
-	golang.org/x/text v0.40.0 // indirect
+	golang.org/x/text v0.41.0 // indirect
 	golang.org/x/time v0.15.0 // indirect
-	golang.org/x/tools v0.47.0 // indirect
+	golang.org/x/tools v0.48.0 // indirect
 	gomodules.xyz/jsonpatch/v2 v2.4.0 // indirect
 	google.golang.org/genproto v0.0.0-20260319201613-d00831a3d3e7 // indirect
 	google.golang.org/genproto/googleapis/api v0.0.0-20260630182238-925bb5da69e7 // indirect