Implement real Seriatim merge stage

internal/app/runner.go

@@ -59,7 +59,11 @@ func executeStages(ctx context.Context, cfg *config.Config, stages []stage.Stage
 		env.WhisperX = client
 	}
 	if env.Seriatim == nil {
-		env.Seriatim = &seriatim.NoopRunner{}
+		runner, err := buildDefaultSeriatimRunner(env.Config)
+		if err != nil {
+			return nil, fmt.Errorf("initialize seriatim runner: %w", err)
+		}
+		env.Seriatim = runner
 	}
 	if env.Audita == nil {
 		env.Audita = &audita.NoopRunner{}
@@ -179,6 +183,40 @@ func buildDefaultWhisperXClient(cfg *config.Config) (whisperx.Client, error) {
 	return client, nil
 }
 
+func buildDefaultSeriatimRunner(cfg *config.Config) (seriatim.Runner, error) {
+	if cfg == nil || cfg.Pipeline == nil {
+		return &seriatim.NoopRunner{}, nil
+	}
+
+	s := cfg.Pipeline.Seriatim
+	if strings.TrimSpace(s.Binary) == "" || strings.TrimSpace(s.Timeout) == "" || strings.TrimSpace(s.OutputSchema) == "" {
+		// Compatibility fallback for tests or internal call paths that bypass config validation/defaults.
+		return &seriatim.NoopRunner{}, nil
+	}
+
+	report := false
+	if s.Report != nil {
+		report = *s.Report
+	}
+	runner, err := seriatim.NewSubprocessRunnerFromConfigValues(
+		s.Binary,
+		s.Timeout,
+		s.OutputSchema,
+		s.CoalesceGap,
+		report,
+		seriatim.EnvConfig{
+			OverlapWordRunGap:           s.Env.OverlapWordRunGap,
+			OverlapWordRunReorderWindow: s.Env.OverlapWordRunReorderWindow,
+			BackchannelMaxDuration:      s.Env.BackchannelMaxDuration,
+			FillerMaxDuration:           s.Env.FillerMaxDuration,
+		},
+	)
+	if err != nil {
+		return nil, fmt.Errorf("from pipeline.seriatim: %w", err)
+	}
+	return runner, nil
+}
+
 func loadOrCreateManifest(ctx context.Context, store manifest.Store, path, sessionID string) (*manifest.Manifest, error) {
 	exists, err := fileExists(path)
 	if err != nil {