fix(nwsync): write emit and assemble summaries to stderr (#82)

Fixes #81.

`crucible nwsync emit` and `assemble` printed their summary line to **stdout**. Any caller that captures a script's stdout as a value gets the summary glued onto it — `pack-haks.sh` does `release_dir="$(...)"`, so all 11 emit summaries landed in `$release_dir` and `publish-release.sh` died with "release dir not found".

Both lines move to stderr, where `lib.sh`'s own `nwsync: emitted $key` log already goes. Neither line is a machine-readable contract: sow-topdata's contract tests grep an `EMIT_LOG` their own fake-crucible stub writes, not real stdout, so nothing parses these.

`runEmit`/`runAssemble` no longer take the stdout writer — a leak in those two functions is now impossible to write by accident. `Run` still passes stdout to `printRunUsage` for explicit `-h`/`help`, which is correct.

Adds `TestRunKeepsSummariesOffStdout`: runs both verbs end to end against a local tree and asserts stdout stays empty while the summary reaches stderr. It asserts emptiness, not wording, so the summary text stays free to change.

Full `go test ./...` green.

Follow-up, outside this repo: sow-assets-manifest needs a `flake.lock` bump, then a re-run of the v0.2.1-rc1 tag.

🤖 Generated with [Claude Code](https://claude.com/claude-code)Reviewed-on: #82
Reviewed-by: xtul <mpiasecki720@protonmail.com>
Co-authored-by: vickydotbat <vickydotbat@tutamail.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,