chore(nodes): run the master+node scopes in parallel and document both layers

Each enrollment scope owns its panels, ports and temp dirs, so the two run
side by side; the only shared state, the process-global database handle the
harness borrows for setup and for simulating a lost inbound, is now behind a
mutex. On Linux the suite drops from 188s to 95s.

CLAUDE.md now names the fast contract layer (node_contract_test.go, in
make test-go) next to the multi-tick nodee2e layer.

internal/nodee2e/node_sync_test.go

@@ -16,6 +16,7 @@ func TestNodeSync(t *testing.T) {
 	bin := panelBinary(t)
 	for _, scope := range []string{"admin", "node-sync"} {
 		t.Run("enrolled with "+scope+" token", func(t *testing.T) {
+			t.Parallel()
 			runNodeSyncScenarios(t, bin, scope)
 		})
 	}