Avoid redundant TUI redraws while streaming

[copyberry]

Avoid redundant TUI redraws while streaming

What changed

• Redraw assistant and plan stream tails only when a completed line changes the visible tail.
• Cache the first reasoning header and update the status indicator only when its visible state changes, while restoring the cached header if the indicator is recreated.
• Keep regular commit ticks able to clear stale stream tails.

Testing

• Add coverage for unchanged partial deltas, changed newline-delimited tails, reasoning-header redraws, recreated status indicators, stale plan tails, and history-line draw scheduling.