feat: route stereo video through eye cameras

[juelg]

Summary

• add dedicated IRISStereoLeft and IRISStereoRight Unity layers
• enable the existing left/right eye anchor cameras in IRISScene.unity
• route those cameras so they render only the matching stereo layer
• exclude the stereo layers from the center eye camera
• pin com.intuitiverobotslab.iris-viz to the companion stereo-surface commit

Why

The stereo imagery should not rely on a custom eye-mask UI shader. The more standard path is to keep the mono window UI on the center camera and let left/right eye cameras render only the eye-specific stereo surfaces.

Companion dependency

This PR expects IRIS-Viz commit f59927ca446f0b58ffdc78539ae873fb082ec561 from the companion PR branch and pins that exact hash in Packages/packages-lock.json.