restic: RunCheck with subset% + lock-state sniffing

Add CheckResult (LockPresent, ErrorsFound) and RunCheck.  subsetPct>0
passes --read-data-subset N% to limit data reads.  Stderr is sniffed
for "Found stale lock"/"locked" to set LockPresent; a non-zero exit
from restic is absorbed as ErrorsFound=true rather than an error so
the caller can always persist last_check_status.  Tests cover lock
detection, exit-1 absorption, and subset-arg plumbing.

internal/restic/runner_test.go

@@ -34,7 +34,7 @@ func captureLines() (*[]string, LineHandler) {
 	return &lines, h
 }
 
-// --- B1: RunPrune ---
+// --- B1: RunPrune + B2: RunCheck ---
 
 func TestRunPruneInvokesPrune(t *testing.T) {
 	// Shell script that echoes its args; "prune" should appear in output.
@@ -51,3 +51,48 @@ func TestRunPruneInvokesPrune(t *testing.T) {
 	}
 	t.Fatalf("expected 'prune' in captured output; got: %v", *lines)
 }
+
+// --- B2: RunCheck ---
+
+func TestRunCheckParsesLock(t *testing.T) {
+	bin := setupScriptBin(t, `echo "Found stale lock" >&2`)
+	env := Env{Bin: bin}
+	res, err := env.RunCheck(context.Background(), 0, nil)
+	if err != nil {
+		t.Fatalf("RunCheck returned unexpected error: %v", err)
+	}
+	if !res.LockPresent {
+		t.Fatal("expected LockPresent=true")
+	}
+	if res.ErrorsFound {
+		t.Fatal("expected ErrorsFound=false")
+	}
+}
+
+func TestRunCheckErrorsFoundOnExit1(t *testing.T) {
+	bin := setupScriptBin(t, `exit 1`)
+	env := Env{Bin: bin}
+	res, err := env.RunCheck(context.Background(), 0, nil)
+	if err != nil {
+		t.Fatalf("RunCheck returned unexpected error (should have absorbed exit 1): %v", err)
+	}
+	if !res.ErrorsFound {
+		t.Fatal("expected ErrorsFound=true for exit 1")
+	}
+}
+
+func TestRunCheckSubsetArg(t *testing.T) {
+	bin := setupScriptBin(t, `echo "$@"`)
+	env := Env{Bin: bin}
+	lines, h := captureLines()
+	if _, err := env.RunCheck(context.Background(), 25, h); err != nil {
+		t.Fatalf("RunCheck: %v", err)
+	}
+	want := "--read-data-subset 25%"
+	for _, l := range *lines {
+		if strings.Contains(l, want) {
+			return
+		}
+	}
+	t.Fatalf("expected %q in captured output; got: %v", want, *lines)
+}