Add subprocess runner and generated config helpers

internal/adapters/seriatim/fake.go

@@ -1,6 +1,11 @@
 package seriatim
 
-import "context"
+import (
+	"context"
+	"fmt"
+
+	"gitea.maximumdirect.net/eric/narratio/internal/adapters/subprocess"
+)
 
 // NoopRunner is a deterministic no-op seriatim adapter.
 type NoopRunner struct{}
@@ -10,6 +15,9 @@ func (n *NoopRunner) Run(ctx context.Context, req MergeRequest) (MergeResult, er
 	if err := ctx.Err(); err != nil {
 		return MergeResult{}, err
 	}
+	if err := materializePlaceholders(req); err != nil {
+		return MergeResult{}, err
+	}
 	return MergeResult{MergedTranscriptPath: req.OutputMergedTranscriptPath, Metadata: map[string]any{"placeholder": true}}, nil
 }
 
@@ -29,6 +37,9 @@ func (f *FakeRunner) Run(ctx context.Context, req MergeRequest) (MergeResult, er
 	if f.Err != nil {
 		return MergeResult{}, f.Err
 	}
+	if err := materializePlaceholders(req); err != nil {
+		return MergeResult{}, err
+	}
 	res := f.Result
 	if res.MergedTranscriptPath == "" {
 		res.MergedTranscriptPath = req.OutputMergedTranscriptPath
@@ -38,3 +49,28 @@ func (f *FakeRunner) Run(ctx context.Context, req MergeRequest) (MergeResult, er
 	}
 	return res, nil
 }
+
+func materializePlaceholders(req MergeRequest) error {
+	if req.GeneratedConfigPath != "" {
+		payload := map[string]any{
+			"schema":                 "seriatim.generated.v1",
+			"placeholder":            true,
+			"input_transcript_paths": req.InputTranscriptPaths,
+			"output_path":            req.OutputMergedTranscriptPath,
+		}
+		if err := subprocess.WriteYAMLAtomic(req.GeneratedConfigPath, payload, 0o644); err != nil {
+			return fmt.Errorf("write generated config %q: %w", req.GeneratedConfigPath, err)
+		}
+	}
+	if req.StdoutLogPath != "" {
+		if err := subprocess.WriteFileAtomic(req.StdoutLogPath, []byte("seriatim noop/fake stdout placeholder\n"), 0o644); err != nil {
+			return fmt.Errorf("write stdout log %q: %w", req.StdoutLogPath, err)
+		}
+	}
+	if req.StderrLogPath != "" {
+		if err := subprocess.WriteFileAtomic(req.StderrLogPath, []byte("seriatim noop/fake stderr placeholder\n"), 0o644); err != nil {
+			return fmt.Errorf("write stderr log %q: %w", req.StderrLogPath, err)
+		}
+	}
+	return nil
+}