Add CultCache geometry documents

src/GameCult.Geometry/README.md

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+# GameCult.Geometry
+
+`GameCult.Geometry` defines CultCache-native documents for procedural geometry
+domains, LOD build requests, selected-cut manifests, and mesh chunk artifacts.
+
+The package is deliberately substrate-shaped:
+
+- CultCache owns document identity, schema compatibility, references, and local
+  persistence.
+- CultNet can distribute these documents through the normal raw document lane.
+- CultMesh can use them as worker-visible shared state for simulation and
+  geometry pipelines.
+- JSON is not a runtime transport here. It may publish schema descriptions at
+  the xeno boundary, but the live machine eats and emits typed CultCache
+  documents.
+