cmd/cli: stop the replacement before rolling its binary back

Rollback ran os.Remove(bin) while the replacement service was still running from
that image. Windows locks a running executable, so the remove failed with
"Access is denied" - and it was fatal, so the os.Rename that restores the
previous binary never ran. The upgrade ended with the broken replacement still
installed and the working binary stranded at its _previous name.

Readiness failing is not evidence the process exited: the service manager can
report a started service whose process never became operational. So rollback now
stops the service and waits until the manager reports it stopped before touching
the executable, then cleans up DNS the way the restart path's Cleanup task does.

Restoring is now conditional on the previous binary reporting a version, since a
_previous file that exists but produces no version output would trade a service
that starts and hangs for one that cannot start at all. When it is unusable,
rollback keeps it for inspection, leaves the installed binary alone, and says so
instead of pressing on. The --version probe is bounded by a timeout so a binary
that hangs cannot hang the upgrade.

Remaining failures are reported rather than fatal, so each one says what state
the host was left in. os.Remove is retried while the path stays locked, since
Windows releases an image lock asynchronously after the process exits.

The helpers live in a new file rather than in commands.go, and the rollback is
extracted into rollbackToPreviousBinary() so it can be covered: the stop happens
while the executable is still present, an unusable previous binary is kept
without swapping or restarting, and a failed stop aborts before anything is
modified. Reversing the stop and the remove fails these tests.

The version probe is called through a variable so those tests do not have to
stage a runnable executable. Staging one is not portable: oldBin is
bin+"_previous", so a fixture named "ctrld" yields the extension-less
"ctrld_previous", which Windows refuses to execute, and a symlink to the test
binary needs a privilege Windows does not grant by default. The probe itself is
still covered against the real test binary. Production is unaffected: ctrld.exe
_previous does have an extension, and os/exec only appends PATHEXT entries when
a path has none at all - noted at binaryVersion so the suffix is not renamed
into something extension-less by accident.

cmd/cli/main_test.go

@@ -1,6 +1,7 @@
 package cli
 
 import (
+	"fmt"
 	"os"
 	"os/exec"
 	"strings"
@@ -11,7 +12,27 @@ import (
 
 var logOutput strings.Builder
 
+// envFakeVersionOutput makes this test binary impersonate a ctrld executable: when
+// set, the process writes the value to stdout and exits without running any test, so
+// binaryVersion() can be exercised on every platform without building or shipping a
+// fixture binary. The value envFakeVersionSilent produces no output at all, which
+// reproduces a ctrld.exe_previous that exists but reports no version.
+//
+// This must be handled before m.Run(), which is what parses the test flags: the child
+// is invoked as "<binary> --version" and would otherwise die on an unknown flag.
+const (
+	envFakeVersionOutput = "CTRLD_TEST_FAKE_VERSION_OUTPUT"
+	envFakeVersionSilent = "<silent>"
+)
+
 func TestMain(m *testing.M) {
+	if out := os.Getenv(envFakeVersionOutput); out != "" {
+		if out != envFakeVersionSilent {
+			fmt.Println(out)
+		}
+		os.Exit(0)
+	}
+
 	l := zerolog.New(&logOutput)
 	mainLog.Store(&l)