Add GPU resident grid GICP covariance preparation

CHANGELOG.md

@@ -21,6 +21,14 @@ removed no sooner than the next major (see `docs/API_STABILITY.md`).
 
 ### Changed
 
+- Add explicit resident grid GICP covariance preparation under
+  `register-gicp-gpu`. Reuse original GPU positions and canonical radius-grid
+  arithmetic, eliminating source KD-tree preparation, per-point neighborhood
+  buffers and covariance readback/re-upload. CPU counting-sort grids and target
+  tree remain; support capacity includes self/duplicates and overflow rejects
+  all equations. Grid indices use private leased scratch with full receipts.
+  Existing sparse/resident/legacy routes retain their explicit contracts.
+
 - Add explicit resident radius covariance preparation for GICP under
   `register-gicp-gpu`. Inclusive bounded GPU neighborhoods, centered/scaled
   compensated central sums and plane eigenvectors feed resident equations

README.md

@@ -681,6 +681,15 @@ Sparse neighborhoods retain the grid route's epsilon*I fallback. CPU tree
 construction and 6x6 solves remain; only 256-byte equations are downloaded.
 See [resident radius evidence](notes/2026-10-13_gpu_gicp_resident_radius_covariances.md).
 
+`align_with_gpu_resident_grid_covariances(&runtime, &source, &target,
+neighbor_capacity)` uses the existing radius-grid covariance math directly on
+resident original positions. CPU builds counting-sort grids and the target
+tree; no source KD-tree or per-point neighborhood buffer is needed. Capacity
+includes self/duplicates and overflow rejects all output. Dense grids must fit
+the existing cell/device limits; choose the bounded radius method explicitly
+for sparse extents. Only 256-byte equations are downloaded per evaluation.
+See [resident grid evidence](notes/2026-10-13_gpu_gicp_resident_grid_covariances.md).
+
 Point-to-plane ICP also exposes
 `PointToPlaneIcp::align_with_gpu_reduction(&runtime, &source, &target)` under
 `register-icp-point-to-plane-gpu`. Transforms, nearest matching and plane