Print --version to stdout (closes #15)

Cobra's built-in version flag prints through the writer that carries
help and usage, which is stderr here. The root command now defines
its own -v/--version flag and prints one line, "sfdupes VERSION", to
stdout; a failed write is a fatal error (exit 1). Help and usage stay
on stderr. README documents --version and --help, their streams and
exit codes. Tests cover both flags and the failed write.

Model: opus-5-5

main_test.go

@@ -295,34 +295,72 @@ func TestRunUsageErrors(t *testing.T) {
 	}
 }
 
-// TestRunHelpAndVersionSucceed checks that the two informational flags
-// exit 0 and keep their human-facing output on stderr.
-func TestRunHelpAndVersionSucceed(t *testing.T) {
+func TestRunHelp(t *testing.T) {
 	t.Parallel()
 
-	assertHumanOutput(t, "--help")
-	assertHumanOutput(t, "--version")
+	// README §Subcommands: help goes to stderr, exits 0, and leaves
+	// stdout empty.
+	cases := [][]string{{"--help"}, {"-h"}, {cmdScan, "--help"}}
+
+	for _, args := range cases {
+		var stdout, stderr bytes.Buffer
+
+		code := run(args, &stdout, &stderr)
+		if code != exitOK {
+			t.Errorf("run(%v) = %d, want %d", args, code, exitOK)
+		}
+
+		if !strings.Contains(stderr.String(), usageMarker) {
+			t.Errorf("run(%v) stderr = %q, want the help text",
+				args, stderr.String())
+		}
+
+		if got := stdout.String(); got != "" {
+			t.Errorf("run(%v) stdout = %q, want nothing (data only)",
+				args, got)
+		}
+	}
 }
 
-// assertHumanOutput runs sfdupes with one informational flag and checks
-// that it succeeds with its output on stderr and stdout untouched
-// (README design goal 4).
-func assertHumanOutput(t *testing.T, arg string) {
-	t.Helper()
+func TestRunVersion(t *testing.T) {
+	t.Parallel()
 
-	var stdout, stderr bytes.Buffer
+	// README §Subcommands: the version is one line on stdout, with
+	// nothing on stderr, and exits 0.
+	for _, arg := range []string{"--version", "-v"} {
+		var stdout, stderr bytes.Buffer
 
-	code := run([]string{arg}, &stdout, &stderr)
-	if code != exitOK {
-		t.Errorf("run(%s) = %d, want %d", arg, code, exitOK)
+		code := run([]string{arg}, &stdout, &stderr)
+		if code != exitOK {
+			t.Errorf("run(%s) = %d, want %d", arg, code, exitOK)
+		}
+
+		want := "sfdupes " + Version + "\n"
+		if got := stdout.String(); got != want {
+			t.Errorf("run(%s) stdout = %q, want %q", arg, got, want)
+		}
+
+		if got := stderr.String(); got != "" {
+			t.Errorf("run(%s) stderr = %q, want nothing", arg, got)
+		}
+	}
+}
+
+func TestRunVersionWriteFailureIsFatal(t *testing.T) {
+	t.Parallel()
+
+	// README §Error handling: a stdout write failure exits 1, reported
+	// in one line on stderr.
+	var stderr bytes.Buffer
+
+	code := run([]string{"--version"}, failingWriter{}, &stderr)
+	if code != exitFatal {
+		t.Errorf("run(--version) = %d, want %d", code, exitFatal)
 	}
 
-	if stderr.Len() == 0 {
-		t.Errorf("run(%s) wrote nothing to stderr", arg)
-	}
-
-	if got := stdout.String(); got != "" {
-		t.Errorf("stdout = %q, want nothing (data only)", got)
+	want := "sfdupes: write stdout: " + errWriteFailed.Error() + "\n"
+	if got := stderr.String(); got != want {
+		t.Errorf("stderr = %q, want %q", got, want)
 	}
 }