[codex] Add jelly demos and improve JellySwitch performance

[jamesrochabrun]

Summary

• Include the full jroch-cards branch context needed by the demo target, including MotionManager, the metal avatar/cosmic card demo work, and JellySlider.
• Add the JellySlider shader, ShaderKitUI component, and demo wiring.
• Pause the JellySwitch animation timeline once the spring state has settled so it stops rendering every frame while idle.
• Call the JellySwitch shader directly from the component to avoid the generic shader modifier's extra drawing pass.
• Add conservative Metal fast paths for flat ground pixels and rays that cannot intersect the jelly volume.

Why

The JellySwitch optimization depends on the active branch context rather than bare main: the demo target on main was missing support files such as MotionManager, so the narrower PR did not represent the branch that actually compiled locally. This PR now carries the full work from jroch-cards plus the JellySwitch performance commit.

Validation

• swift build passes.
• xcodebuild -project ShaderKitDemo.xcodeproj -scheme ShaderKitDemo -destination 'generic/platform=iOS Simulator' build passes.
• git diff --check origin/main..HEAD passes.
• swift test currently fails on an existing version assertion: ShaderKit.version is 2.0.0, while ShaderKitTests.swift expects 1.0.0.