kvm2: integration test, sample Proxy, providers-config entries and Makefile target

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

config/samples/proxy_kvm2.yaml

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+# A Managed proxy backed by a libvirt/KVM hypervisor through the kvm2
+# provider: the operator boots a stock Debian cloud image as a VM on the
+# named host, with a NIC on the host's bridge (IP from the LAN's DHCP), and
+# installs Squid via cloud-init. Requires a providers-config entry named
+# "kvm2-home" (see providers-config.yaml) and the kvm-ssh Secret, which the
+# kvm and kvm2 providers share (docs/kvm2-provider.md).
+#
+# placement.zone is the host name from kvm2.hosts (optional when only one
+# host is configured); machineType and image are keys in the provider's
+# machineTypes and images maps. The first Proxy for an image on a host
+# waits for the image download; later ones reuse the cached copy.
+apiVersion: crawl.example.com/v1alpha1
+kind: Proxy
+metadata:
+  name: proxy-kvm2-sample
+spec:
+  mode: Managed
+  provider: kvm2-home
+  placement:
+    zone: beelink
+    machineType: small
+    image: debian-13
+  port: 3128
+  cloudInit:
+    inline: |
+      #cloud-config
+      packages:
+        - squid
+      write_files:
+        - path: /etc/squid/conf.d/proxy-operator.conf
+          content: |
+            http_port 3128
+            http_access allow all
+            via off
+            forwarded_for off
+      runcmd:
+        - systemctl restart squid
+      # Optional: log in with `ssh debian@<ip>` (the Debian image's default
+      # user) to look around; `virsh console <name>` on the host works too.
+      # ssh_authorized_keys:
+      #   - ssh-ed25519 AAAA... you@laptop
+  attributes:
+    geo: home
+    purpose: crawl