perf(client): sync CEF frames to render tick

[godperkys]

Summary

• enable external begin-frame scheduling for windowless CEF browsers
• send external begin frames from the game render tick for visible browsers
• skip hidden browser paint work and clear hidden pending paints
• use CEF dirty rects for overlay texture updates to avoid full texture uploads when only small HUD regions change

Why

CEF offscreen rendering was still paced by Chromium's internal frame scheduling, which can make active HUD/browser surfaces feel capped around 60 FPS while the game is rendering faster. Driving visible browsers from the render tick keeps browser updates aligned with the game frame loop.

Validation

• cmake --build build\client-verify --config Debug --target Client Renderer Loader