feat(tui): added component-scoped rendering with partial compose reuse

- Added `TUI.requestComponentRender`, enabling component-scoped render requests.
- Implemented partial compose reuse by resolving and memoizing affected root segments.
- Fixed component-scoped updates to keep frame-seam rows continuous across partial renders.
- Added component-render and loader tests for reuse, downgrade, and call-count coverage.

packages/coding-agent/src/modes/components/btw-panel.ts

@@ -73,7 +73,11 @@ export class BtwPanelComponent extends Container {
 		this.addChild(new Text(this.#footerLine(), 1, 0));
 		this.addChild(new Spacer(1));
 		this.addChild(new DynamicBorder(str => theme.fg("dim", str)));
-		this.#tui.requestRender();
+		// Component-scoped: a rebuild replaces only this panel's own children
+		// (streaming deltas arrive per token, and a full compose would re-walk
+		// the whole transcript each time). Before the panel is mounted the TUI
+		// cannot resolve it and falls back to a full compose on its own.
+		this.#tui.requestComponentRender(this);
 	}
 
 	#footerLine(): string {