Adaptive live-video: skip near-duplicate frames, lower FPS on static scenes

[rayl15]

Part of the power-management roadmap in #36.

Goal

Reduce wasted work during Live video mode by adapting to scene content.

Scope

• Skip near-duplicate frames (cheap frame-difference check before sending to the model)
• Lower frame rate when the scene is static; raise it when motion/detail warrants
• Configurable frame rate / resolution ceilings
• Prefer a single high-res still over sustained high-quality video where the task allows

Pointers

• OpenVision/Managers/GlassesManager.swift — frame publishing
• OpenVision/Views/VoiceAgent/VoiceAgentView.swift — freshestGlassesFrame / live-video send loop

Higher effort than #36's cleanup slice — good for someone comfortable with the streaming path.