Define adapter interfaces and fake implementations

internal/adapters/seriatim/fake.go

@@ -0,0 +1,40 @@
+package seriatim
+
+import "context"
+
+// NoopRunner is a deterministic no-op seriatim adapter.
+type NoopRunner struct{}
+
+// Run returns the requested output path with placeholder metadata.
+func (n *NoopRunner) Run(ctx context.Context, req MergeRequest) (MergeResult, error) {
+	if err := ctx.Err(); err != nil {
+		return MergeResult{}, err
+	}
+	return MergeResult{MergedTranscriptPath: req.OutputMergedTranscriptPath, Metadata: map[string]any{"placeholder": true}}, nil
+}
+
+// FakeRunner captures merge requests and returns deterministic responses.
+type FakeRunner struct {
+	Requests []MergeRequest
+	Err      error
+	Result   MergeResult
+}
+
+// Run records request and returns configured response.
+func (f *FakeRunner) Run(ctx context.Context, req MergeRequest) (MergeResult, error) {
+	if err := ctx.Err(); err != nil {
+		return MergeResult{}, err
+	}
+	f.Requests = append(f.Requests, req)
+	if f.Err != nil {
+		return MergeResult{}, f.Err
+	}
+	res := f.Result
+	if res.MergedTranscriptPath == "" {
+		res.MergedTranscriptPath = req.OutputMergedTranscriptPath
+	}
+	if res.Metadata == nil {
+		res.Metadata = map[string]any{"fake": true}
+	}
+	return res, nil
+}