Keep _EMISSION/_METALLICSPECGLOSSMAP/_OCCLUSIONMAP shader variants alive in builds

Add a Resources material that sets all three PBR shader_feature keywords so
core build stripping retains the URP/Lit variants the runtime mesh painter
enables on the fly. Fixes metallic/smoothness/occlusion rendering as constant
in player builds.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Assets/Resources/PaintVariantKeepAliveTex.asset

@@ -0,0 +1,48 @@
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