controller v0.267.0: tests off DooPlex's Docker, cut-off copies refused, two pages true

R-650: internal/dockerexec — every docker exec routed through it; under
go test a real docker is refused (opt-in FELHOM_TEST_REAL_DOCKER=1; a stub
under the temp dir is allowed). api/stacks/web tests run under a silent
stub (TestMain). TestR650_NoBareDockerExec pins it repo-wide.
R-640: a dump without its engine's completion marker is refused before
the first mutation (unit + off-site restore) and again before any load.
R-499: the Tier-2 page's system-disk sentence has four true branches.
R-518: the backup button states the measured ~8 min stop.
R-626: measured on 9202, not reproduced.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_0159rPz1ZhFKsS53msqPYxtS

controller/internal/stacks/guestnet.go

@@ -2,8 +2,8 @@ package stacks
 
 import (
 	"fmt"
+	"gitea.dooplex.hu/admin/felhom-controller/internal/dockerexec"
 	"net"
-	"os/exec"
 	"strings"
 )
 
@@ -45,7 +45,7 @@ func (m *Manager) guestNetExec(args ...string) (string, error) {
 	if m.guestNetExecFn != nil {
 		return m.guestNetExecFn(args...)
 	}
-	out, err := exec.Command("docker", append([]string{"exec", sambaContainer}, args...)...).Output()
+	out, err := dockerexec.Command("docker", append([]string{"exec", sambaContainer}, args...)...).Output()
 	if err != nil {
 		return "", fmt.Errorf("docker exec %s: %w", strings.Join(args, " "), err)
 	}