Trait injection: dark mode, dynamic type, device sizes

[obj-p]

Add a preview_configure MCP tool and CLI flags for injecting SwiftUI environment traits:

• Color scheme: light/dark mode via .preferredColorScheme
• Dynamic type: accessibility sizes via .dynamicTypeSize
• Device frame sizes (width/height overrides)

Example: preview_configure(sessionID, colorScheme: "dark", dynamicType: "large")

From the roadmap in tidy-dreaming-stream.md. The Xcode trace data in docs/build-system-integration.md notes that Xcode uses renderEffects in the preview specification payload for trait overrides — investigating this format could inform the implementation.