kvm2: config block, constructor registration, IDs, state mapping and classify

The kvm2 block reuses KVMConfig; validateKVM gains a block-name variant so
messages say kvm2. The package skeleton has a non-dialing New, strict
providerID parsing, the state table and a 34-row classify test.

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

internal/provider/config_test.go

@@ -118,6 +118,26 @@ providers:
       image: custom-image`,
 			wantErrSub: "type is gcp but a kubernetes block is set",
 		},
+		{
+			name: "kvm type with kvm2 block",
+			yaml: `
+providers:
+  - name: p1
+    type: kvm
+    kvm2:
+      sshKeyFile: /key`,
+			wantErrSub: "type is kvm but a kvm2 block is set",
+		},
+		{
+			name: "kvm2 type with kvm block",
+			yaml: `
+providers:
+  - name: p1
+    type: kvm2
+    kvm:
+      sshKeyFile: /key`,
+			wantErrSub: "type is kvm2 but a kvm block is set",
+		},
 		{
 			name: "strict mode rejects unknown top-level key",
 			yaml: `