feat: enhance visual capture capabilities for macOS and web

- Introduced weak `Texture` tier with advisory for `desktop_window` mode.
- Added `MCPToolkitBinding.captureHintsContributor` for WGPU/custom-engine apps.
- Enhanced `get_screenshots` to include routing metadata and leading JSON block.
- Implemented macOS host capture for Chrome/web targets using ScreenCaptureKit.
- Updated documentation to reflect changes in platform view capture routing and hints.