Add 3DGS columnar splat codec plan

Author: AdaWorldAPI

.claude/plans/3DGS-columnar-splat-codec-plan.md

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+# 3DGS Columnar Splat Codec Plan — ndarray
+
+## Goal
+
+Define the ndarray-side columnar representation and codec primitives for 3DGS splat blocks.
+
+The representation must be friendly to:
+
+- SIMD batch projection
+- Arrow/Lance sidecar storage in `lance-graph`
+- quantization and compression
+- deterministic validation
+- future glTF/SPZ/3D Tiles import-export bridges
+
+## Non-goals
+
+- Do not implement 3D Tiles package writing here.
+- Do not own metadata schemas for full geospatial tilesets.
+- Do not implement a server.
+
+## Core representation
+
+Prefer struct-of-arrays for hot paths:
+
+```rust
+pub struct SplatBlockColumns<T> {
+    pub pos_x: Vec<T>,
+    pub pos_y: Vec<T>,
+    pub pos_z: Vec<T>,
+    pub scale_x: Vec<T>,
+    pub scale_y: Vec<T>,
+    pub scale_z: Vec<T>,
+    pub quat_w: Vec<T>,
+    pub quat_x: Vec<T>,
+    pub quat_y: Vec<T>,
+    pub quat_z: Vec<T>,
+    pub opacity: Vec<T>,
+    pub color_rgba: Vec<[u8; 4]>,
+    pub feature_id: Vec<u32>,
+}
+```
+
+For zero-copy interop, expose borrowed views:
+
+```rust
+pub struct SplatBlockView<'a, T> {
+    pub pos_x: &'a [T],
+    pub pos_y: &'a [T],
+    pub pos_z: &'a [T],
+    pub scale_x: &'a [T],
+    pub scale_y: &'a [T],
+    pub scale_z: &'a [T],
+    pub quat_w: &'a [T],
+    pub quat_x: &'a [T],
+    pub quat_y: &'a [T],
+    pub quat_z: &'a [T],
+    pub opacity: &'a [T],
+    pub color_rgba: &'a [[u8; 4]],
+    pub feature_id: &'a [u32],
+}
+```
+
+## Quantization tiers
+
+Implement explicit tiers, not hidden magic:
+
+1. `F32Raw`
+   - reference format
+   - easiest validation
+
+2. `F16Packed`
+   - scale/quaternion/opacity where tolerable
+   - use existing f16 carrier machinery
+
+3. `Bf16Packed`
+   - wider exponent for geospatial scale variation
+   - useful for world-space values after local origin rebasing
+
+4. `I16LocalTile`
+   - local tile coordinate frame
+   - positions stored as i16/u16 with scale/offset
+
+5. `PaletteBlock`
+   - optional future tier for repeated covariance/color/SH patterns
+
+## Tile-local coordinate policy
+
+World geospatial coordinates should not be projected directly in f32 at large scales.
+
+Use:
+
+```text
+ECEF / projected world coordinate
+        ->
+tile local origin
+        ->
+local f32 or quantized i16 block coordinate
+```
+
+The codec should support a `TileLocalFrame`:
+
+```rust
+pub struct TileLocalFrame {
+    pub origin_world: [f64; 3],
+    pub axis_x: [f64; 3],
+    pub axis_y: [f64; 3],
+    pub axis_z: [f64; 3],
+    pub scale: f64,
+}
+```
+
+## Stats emitted by codec
+
+Every encode/decode must produce stats:
+
+```rust
+pub struct SplatCodecStats {
+    pub splat_count: usize,
+    pub bytes_raw: usize,
+    pub bytes_encoded: usize,
+    pub max_position_error: f32,
+    pub rms_position_error: f32,
+    pub max_scale_error: f32,
+    pub max_opacity_error: f32,
+    pub invalid_quaternion_count: usize,
+    pub invalid_scale_count: usize,
+}
+```
+
+These stats feed `3DGS-error-certification-pillars-plan.md`.
+
+## Module layout
+
+```text
+src/hpc/splat3d/codec/
+  mod.rs
+  columns.rs
+  local_frame.rs
+  quant_f16.rs
+  quant_bf16.rs
+  quant_i16_tile.rs
+  palette.rs
+  stats.rs
+```
+
+## Acceptance criteria
+
+- Lossless f32 roundtrip test.
+- Quantized roundtrip tests with explicit max/RMS error checks.
+- Tile-local coordinate tests for large ECEF-like origins.
+- SIMD renderer accepts borrowed views without copying.
+- Codec stats can feed certificate generation.
+
+## Cross-repo integration
+
+`lance-graph` should own Arrow/Lance schema and durable storage.
+
+`ndarray` should own the hot-path memory layout and conversion kernels.
+
+Recommended bridge:
+
+```text
+lance-graph Arrow RecordBatch
+        ->
+borrowed ndarray SplatBlockView
+        ->
+projection / certification / report
+```