Publish UnifiedRT into Core: remove LightTransport package, keep 6000.0–6000.3 compatible

[TrevorDaschOculus]

Publish UnifiedRT Step 2: Removing the LightTransport package and putting its content in com.unity.render-pipelines.core

Cherry-picked from 4c2f15f.

Conflict resolution: took the incoming UnifiedRayTracing/Sampling package content wholesale, and dropped the now-removed com.unity.rendering.light-transport dependency from the SRP test project manifests (the package no longer exists on disk).

Also migrated the APV baking consumers (ProbeGIBaking.SkyOcclusion/RenderingLayers/ VirtualOffset/LightTransport) to the new package API so the 6000.0 branch compiles:

• SamplingResources.BindSobolBlueNoiseTextures -> SamplingResources.Bind
• AccelStructAdapter.AddInstance now takes a per-submesh isOpaque span (all true, preserving the previous MeshInstanceDesc.opaqueGeometry default)

Verified compiling on Unity 6000.0.23f1, 6000.0.67f1, 6000.2.5f1, 6000.2.12f1 and 6000.3.2f1.

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