fix(nwsync): write emit and assemble summaries to stderr

crucible nwsync emit and assemble printed their summary line to stdout.
Callers that capture a script's stdout as a value got the summary glued
onto it: pack-haks.sh does release_dir="$(...)", so all 11 emit summaries
landed in $release_dir and publish-release.sh failed with "release dir
not found".

Neither line is a machine-readable contract, so move both to stderr,
where lib.sh's own "nwsync: emitted $key" log already goes. runEmit and
runAssemble no longer need the stdout writer.

Fixes #81.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

internal/nwsync/nwsync_test.go

@@ -458,6 +458,33 @@ func TestAssembleFailsClosedOnMissingIndex(t *testing.T) {
 	}
 }
 
+// Callers capture a script's stdout as a value: `dir="$(pack-haks.sh)"`. A
+// summary line on stdout gets glued onto that value, so both summaries belong
+// on stderr.
+func TestRunKeepsSummariesOffStdout(t *testing.T) {
+	dir := t.TempDir()
+	path := filepath.Join(dir, "sow_top.hak")
+	writeHak(t, path, map[string][]byte{"appearance.2da": []byte("2da from sow_top")})
+	key := artifactKey(t, path)
+	out := filepath.Join(dir, "out")
+
+	for _, args := range [][]string{
+		{"emit", "--out", out, "--as", "sow_top.hak", key, path},
+		{"assemble", "--out", out, key},
+	} {
+		var stdout, stderr bytes.Buffer
+		if code := Run(args, &stdout, &stderr); code != exitOK {
+			t.Fatalf("Run(%v) exit=%d: %s", args, code, stderr.String())
+		}
+		if stdout.Len() != 0 {
+			t.Errorf("Run(%v) wrote to stdout: %q", args, stdout.String())
+		}
+		if stderr.Len() == 0 {
+			t.Errorf("Run(%v) reported no summary on stderr", args)
+		}
+	}
+}
+
 func TestRunUsageErrors(t *testing.T) {
 	cases := [][]string{
 		nil,