Avoid unchanged TUI footer redraws during streaming #27

Problem

During hidden reasoning and streamed tool calls, the TUI repeatedly clears and redraws the footer even when its visible content has not changed. Because the erase and redraw are separate writes, this can appear as flicker.

This issue is complete when:

• A reasoning-only stream writes nothing when the footer is unchanged.
• Calling setActivity again with the same text does not repaint. Only the one-second tick repaints.
• Actual footer changes bracket the complete update with synchronized output.

Increasing the 40 ms interval or adding synchronized output without skipping unchanged frames is not sufficient. Switching to the alternate screen is not a fix.

Reproduction

1. Start an interactive Motif session.
2. Submit a prompt that causes several seconds of reasoning or a streamed tool call.
3. Watch the composer and activity line while their visible text remains unchanged.
4. On affected terminals, the footer repeatedly flashes during the stream.