feat: add domain types for structured streaming (Phase 1)

internal/domain/stream.go

@@ -0,0 +1,24 @@
+package domain
+
+// StreamType distinguishes between partial text content and metadata events.
+type StreamType string
+
+const (
+	StreamTypeContent StreamType = "content"
+	StreamTypeUsage   StreamType = "usage"
+	StreamTypeError   StreamType = "error"
+)
+
+// StreamUpdate is the unified payload sent through the internal channels.
+type StreamUpdate struct {
+	Type    StreamType     `json:"type"`
+	Content string         `json:"content,omitempty"` // For text deltas
+	Usage   *UsageMetadata `json:"usage,omitempty"`   // For token counts
+}
+
+// UsageMetadata normalizes token counts across different providers.
+type UsageMetadata struct {
+	InputTokens  int `json:"input_tokens"`
+	OutputTokens int `json:"output_tokens"`
+	TotalTokens  int `json:"total_tokens"`
+}