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>

docs/plans-executions/2026-09-04-1404-kvm-provider.md

@@ -4,7 +4,7 @@ Plan: [2026-09-04-1404-kvm-provider.md](../plans/2026-09-04-1404-kvm-provider.md
 
 - [x] Step 0 — Save plan + branch `feat/kvm-provider`
 - [x] Step 1 — Config (`KVMConfig` & friends, `validate()` case, tests)
-- [ ] Step 2 — Pure builders (`domainxml.go`, `volume.go`, `seed.go`, `id.go`, `metadata.go`)
+- [x] Step 2 — Pure builders (`domainxml.go`, `volume.go`, `seed.go`, `id.go`, `metadata.go`)
 - [ ] Step 3 — Transport + errors (`conn.go`, `errors.go`)
 - [ ] Step 3b — Image cache (`image.go`)
 - [ ] Step 4 — Provider methods (`kvm.go`) over a fake API
@@ -52,3 +52,50 @@ in favour of `NewWithDialer(socket.Dialer)`, where `Dialer` is a one-method
 `Dial() (net.Conn, error)` interface. Step 3 should wrap the SSH client's unix
 forward in that interface instead of using `New`; the plan's transport design
 is otherwise unaffected.
+
+## Step 2 — Pure builders
+
+`internal/provider/kvm/` now has the five builders from the plan plus
+`kvm.go` holding the package doc and `withDefaults` (disk size 10 GiB, the
+default vendor-data, `/var/run/libvirt/libvirt-sock`, pool `default`, image
+format `qcow2`). Everything is a pure function over `encoding/xml` structs or
+strings, unit-tested by unmarshalling the output back and checking fields.
+
+Two design choices differ from the plan text:
+
+- **Domain disks reference pool volumes, not host paths.** The plan had
+  `buildDomainXML` take the overlay and seed paths (needing two
+  `StorageVolGetPath` round-trips per Create). The domain XML now uses
+  `<disk type="volume"><source pool=... volume=.../>`, so only the *base*
+  image path is ever looked up (the overlay's `<backingStore>` needs a real
+  path). Fewer RPCs and the builder's inputs are all names.
+- **Capacities are always `unit="bytes"`.** libvirt's `G` means GiB but
+  `GB` means 10^9; spelling the byte count out removes the ambiguity from
+  both the overlay and the seed/base volumes.
+
+The metadata element is written with a default namespace
+(`<instance xmlns="…/kvm">`) because `encoding/xml` cannot emit prefixes on
+marshal. libvirt may hand it back prefixed; `parseInstanceMetadata` is
+tested against both spellings since `encoding/xml` matches on the namespace
+URI. Whether libvirt accepts the default-namespace form on `DomainDefineXML`
+is confirmed in Step 6 against a real host, not here.
+
+Checked before relying on `github.com/kdomanski/iso9660` for the seed: its
+filename mangling lowercases and keeps `-`, so `user-data`/`meta-data`/
+`vendor-data` come out as `user-data;1` etc. in the directory record, which
+the Linux isofs driver (and therefore cloud-init's NoCloud mount) presents as
+`user-data`. The seed test opens the ISO with the same library's reader and
+asserts label `cidata`, all three files, and byte-identical output for
+identical inputs. `NewWriter()` stages files in a temp directory, so
+`Cleanup()` is deferred.
+
+`go mod tidy` ran as part of this step; it promoted `kdomanski/iso9660` to a
+direct dependency and dropped `go-libvirt` again because nothing imports it
+yet — Step 3 brings it back.
+
+Worth noting: `withDefaults` copies the `Images` map and `Hosts` slice
+instead of mutating the config in place, so the registry's shared
+`ProviderConfig` stays untouched. The default vendor-data installs
+`qemu-guest-agent` via `packages:` + `runcmd`, which means the first boot
+needs network access to the distro mirrors; a Proxy on an isolated bridge
+would still work (ARP discovery), just without the agent.