Add 3DGS implementation plan index

Author: AdaWorldAPI

.claude/plans/3DGS-PLAN-INDEX.md

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+# 3DGS Implementation Plan Index — lance-graph
+
+This directory contains the lance-graph-side implementation plans for the 3DGS geospatial rebuild.
+
+## lance-graph responsibility
+
+`lance-graph` owns the orchestration, geospatial contracts, metadata, query layer, and service/runtime wiring:
+
+- Cesium / 3D Tiles feature mapping.
+- 3D Tiles reader/writer/server trajectory.
+- ArcGIS and Cesium source ingestion.
+- Lance/Arrow schemas for tiles, splats, features, metadata, and certificates.
+- Graph/DataFusion/Cypher query interfaces.
+- Integration wiring to `ndarray::hpc::splat3d`, `ndarray::hpc::pillar`, and HHTL kernels.
+- Runtime selection policy, traversal, scheduling, and certified tile decisions.
+
+## Plans
+
+1. `3DGS-Cesium-feature-mapping-plan.md`
+   - Maps Cesium/3D Tiles concepts to Rust/Lance/Arrow/3DGS responsibilities.
+
+2. `3DGS-3D-Tiles-runtime-plan.md`
+   - Defines the 3D Tiles reader/writer/traversal/server crate trajectory.
+
+3. `3DGS-Lance-Arrow-storage-plan.md`
+   - Defines durable columnar schemas for tiles, contents, splats, features, and certificates.
+
+4. `3DGS-integration-wiring-plan.md`
+   - Defines cross-repo DTOs and call flow between `lance-graph` and `ndarray`.
+
+5. `3DGS-ArcGIS-Cesium-ingestion-plan.md`
+   - Defines ingestion from ArcGIS REST, 3D Tiles, glTF, SPZ, GeoJSON, and vector tiles.
+
+6. `3DGS-certified-query-render-plan.md`
+   - Defines the certified render/query decision layer.
+
+7. `3DGS-epiphany-roadmap-plan.md`
+   - Collects additional high-leverage directions discovered during planning.
+
+## Cross-repo boundary
+
+`lance-graph` should not own CPU SIMD renderer internals. It should call stable kernels from `ndarray`.
+
+The intended flow is:
+
+```text
+3D Tiles / ArcGIS / Cesium source
+        ->
+lance-graph ingest + Lance/Arrow metadata
+        ->
+traversal / query / tile decision planning
+        ->
+ndarray 3DGS SIMD + certification kernels
+        ->
+certified render/query decision report
+```
+
+Central principle: keep geospatial orchestration and numerical hot paths separate, but make their DTO boundary explicit and testable.