proxmox: config block, IDs, marks, state mapping, classify, registration

Phase 2 of the proxmox provider plan. Adds the proxmox: config block with
its validation, registers the type in the config allow-list and the
constructor map, and starts internal/provider/proxmox: the api seam and
its data types, providerIDs (vms/<vmid>/<name>) with the UID-derived VMID,
the description mark, state mapping, and error classification.

Tests read the responses captured from pv1-1 in Phase 0 rather than
repeating Proxmox's wording, and every matcher is run against every
captured error so none fires on the wrong message. The contract methods
are Transient stubs until Phases 5 to 7.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

internal/provider/config_test.go

@@ -138,6 +138,26 @@ providers:
       sshKeyFile: /key`,
 			wantErrSub: "type is kvm2 but a kvm block is set",
 		},
+		{
+			name: "proxmox type with kvm block",
+			yaml: `
+providers:
+  - name: p1
+    type: proxmox
+    kvm:
+      sshKeyFile: /key`,
+			wantErrSub: "type is proxmox but a kvm block is set",
+		},
+		{
+			name: "kvm type with proxmox block",
+			yaml: `
+providers:
+  - name: p1
+    type: kvm
+    proxmox:
+      url: https://pve:8006`,
+			wantErrSub: "type is kvm but a proxmox block is set",
+		},
 		{
 			name: "strict mode rejects unknown top-level key",
 			yaml: `