antigravity interactions harness - improve TTFT (per-turn -> per chunk streaming)

[joycel-github]

Problem statement

The Antigravity Interactions harness's output granularity is per turn, coarser than the SDK harness (which is per-Content / block).

Code pointers

• Buffer text deltas into one string:

  ax/internal/harness/antigravityinteractions/antigravityinteractions.go

  Lines 921 to 933 in 6c6bf18

  	case "text":
  	if textChunk, ok := delta["text"].(string); ok {
  	// Text deltas are mostly incremental, but the server
  	// periodically re-sends a full cumulative restatement of the
  	// text so far (e.g. when finalizing a turn before a tool
  	// call). Detect that case and replace rather than append, so
  	// the restated text is not duplicated.
  	if strings.HasPrefix(textChunk, turn.modelText) && turn.modelText != "" {
  	turn.modelText = textChunk
  	} else {
  	turn.modelText += textChunk
  	}
  	}

• Emit once per turn (emitText at the top of the Run loop):

  ax/internal/harness/antigravityinteractions/antigravityinteractions.go

  Lines 377 to 381 in 6c6bf18

  	for turn := 0; turn < e.harness.cfg.MaxTurns; turn++ {
  	e.harness.debugTurn(e.conversationID, turn+1, len(res.toolCalls))
  	if err := emitText(ctx, handler, e.id, res.modelText); err != nil {
  	return err
  	}

• emitText builds a single TextContent per turn:

  ax/internal/harness/antigravityinteractions/antigravityinteractions.go

  Lines 422 to 433 in 6c6bf18

  	// emitText forwards non-empty model text to the handler as a Message.
  	func emitText(ctx context.Context, handler harness.Handler, execID, text string) error {
  	if strings.TrimSpace(text) == "" {
  	return nil
  	}
  	return handler.OnMessage(ctx, execID, &proto.Message{
  	Role: "assistant",
  	Content: &proto.Content{
  	Type: &proto.Content_Text{Text: &proto.TextContent{Text: text}},
  	},
  	})
  	}

Desired behavior

• Emit the first visible output as soon as the SSE stream produces it, rather than buffering the whole turn.
• Stream per chunk / per Content instead of per turn to improve TTFT.
• Keep final/durable content semantics correct.