feat: add DepthPicking component and useDepthPicking hook

[kvvasuu]

Fixes #288

Summary

• New <DepthPicking> component and useDepthPicking hook for reading world-space positions off the depth buffer, extracted from <Autofocus> into a standalone, reusable primitive. <Autofocus> now builds on both internally, same external behavior.
• useDepthPicking auto-detects the composer's own camera when used inside its <EffectComposer>, with an explicit camera argument to override when called from outside that tree.
• Split DepthPicking and N8AO into a new src/passes directory, since neither wraps a postprocessing Effect.

Test plan

• yarn typecheck / yarn test / yarn eslint:ci / yarn build
• Manually verified in a real browser: mouse-follow picking, click-to-pick, <Autofocus> (mouse + debug), self-occlusion (depthWrite), enableNormalPass + surrounding effects, and drei's <PerspectiveCamera>

[jo-chemla]

Thanks for this one, great to hear!