Define adapter interfaces and fake implementations

internal/adapters/seriatim/runner.go

@@ -3,18 +3,22 @@ package seriatim
 
 import "context"
 
-// Runner is a placeholder adapter interface for seriatim invocation.
+// Runner is the adapter boundary for seriatim merge invocations.
 type Runner interface {
 	Run(ctx context.Context, req MergeRequest) (MergeResult, error)
 }
 
-// MergeRequest is a placeholder merge input.
+// MergeRequest describes a seriatim merge invocation.
 type MergeRequest struct {
-	TranscriptPaths []string
-	OutputPath      string
+	GeneratedConfigPath        string
+	InputTranscriptPaths       []string
+	OutputMergedTranscriptPath string
+	StdoutLogPath              string
+	StderrLogPath              string
 }
 
-// MergeResult is a placeholder merge output.
+// MergeResult describes a merge output.
 type MergeResult struct {
-	MergedPath string
+	MergedTranscriptPath string
+	Metadata             map[string]any
 }