Add OverlayOptions for configurable positioning (anchor, margins, offsets,
percentages). Add OverlayHandle for programmatic visibility control with
hide/setHidden/isHidden. Add visible callback for responsive overlays.
Extension API: ctx.ui.custom() now accepts overlayOptions and onHandle callback.
Examples: overlay-qa-tests.ts (10 test commands), doom-overlay (DOOM at 35 FPS).
Renamed from supportsStreamOptions to clarify this controls stream_options: { include_usage: true }.
Defaults to true (no behavioral change for existing providers).
Providers like gatewayz.ai that reject this parameter can set supportsUsageInStreaming: false in model config.
Based on #596 by @XesGaDeus
