Add 3DGS implementation plan index

Author: AdaWorldAPI

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

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+# 3DGS Implementation Plan Index — ndarray
+
+This directory contains the ndarray-side implementation plans for the 3DGS geospatial rebuild.
+
+## ndarray responsibility
+
+`ndarray` owns the numerical and rendering substrate:
+
+- CPU-SIMD 3D Gaussian Splatting forward renderer.
+- SPD covariance construction and EWA projection math.
+- Pillar-certified error probes under `src/hpc/pillar`.
+- Columnar splat payload formats and quantization carriers.
+- HHTL / HEEL-HIP-TWIG-LEAF selection primitives callable by `lance-graph`.
+- Benchmarks, golden vectors, reproducibility, and failure gates.
+
+## Plans
+
+1. `3DGS-SIMD-forward-renderer-plan.md`
+2. `3DGS-error-certification-pillars-plan.md`
+3. `3DGS-columnar-splat-codec-plan.md`
+4. `3DGS-HHTL-CPU-cascade-plan.md`
+5. `3DGS-validation-benchmark-plan.md`
+
+## Cross-repo boundary
+
+`ndarray` should not own 3D Tiles, Cesium compatibility, ArcGIS service ingestion, graph query planning, or tile serving. Those live in `lance-graph`.
+
+The intended interface is:
+
+```text
+lance-graph camera/tile decision request
+        ->
+ndarray HHTL/SIMD/certification kernels
+        ->
+certified tile/splat decision report
+```
+
+Central principle: renderer decisions should be fast, inspectable, and mathematically auditable.