Implement Seriatim subprocess adapter

internal/adapters/seriatim/fake.go

@@ -18,7 +18,15 @@ func (n *NoopRunner) Run(ctx context.Context, req MergeRequest) (MergeResult, er
 	if err := materializePlaceholders(req); err != nil {
 		return MergeResult{}, err
 	}
-	return MergeResult{MergedTranscriptPath: req.OutputMergedTranscriptPath, Metadata: map[string]any{"placeholder": true}}, nil
+	return MergeResult{
+		MergedTranscriptPath: req.OutputMergedTranscriptPath,
+		ReportPath:           req.ReportPath,
+		StdoutLogPath:        req.StdoutLogPath,
+		StderrLogPath:        req.StderrLogPath,
+		GeneratedConfigPath:  req.GeneratedConfigPath,
+		InvokedBinary:        "noop",
+		Metadata:             map[string]any{"placeholder": true},
+	}, nil
 }
 
 // FakeRunner captures merge requests and returns deterministic responses.
@@ -44,6 +52,21 @@ func (f *FakeRunner) Run(ctx context.Context, req MergeRequest) (MergeResult, er
 	if res.MergedTranscriptPath == "" {
 		res.MergedTranscriptPath = req.OutputMergedTranscriptPath
 	}
+	if res.ReportPath == "" {
+		res.ReportPath = req.ReportPath
+	}
+	if res.StdoutLogPath == "" {
+		res.StdoutLogPath = req.StdoutLogPath
+	}
+	if res.StderrLogPath == "" {
+		res.StderrLogPath = req.StderrLogPath
+	}
+	if res.GeneratedConfigPath == "" {
+		res.GeneratedConfigPath = req.GeneratedConfigPath
+	}
+	if res.InvokedBinary == "" {
+		res.InvokedBinary = "fake"
+	}
 	if res.Metadata == nil {
 		res.Metadata = map[string]any{"fake": true}
 	}
@@ -72,5 +95,10 @@ func materializePlaceholders(req MergeRequest) error {
 			return fmt.Errorf("write stderr log %q: %w", req.StderrLogPath, err)
 		}
 	}
+	if req.ReportPath != "" {
+		if err := subprocess.WriteFileAtomic(req.ReportPath, []byte(`{"schema":"seriatim.report.v1","placeholder":true}`), 0o644); err != nil {
+			return fmt.Errorf("write report %q: %w", req.ReportPath, err)
+		}
+	}
 	return nil
 }