skills: add GPU/A2A reference skill documents

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

skills/ref_grid_simt_pattern.md

@@ -0,0 +1,47 @@
+---
+name: ref_grid_simt_pattern
+description: Grid GRID_SIMT
+metadata: 
+  node_type: memory
+  type: reference
+  originSessionId: 956e80aa-401d-481a-80bb-17f8abe1c131
+---
+
+## The problem
+
+On CPU, `accelerator_for(sf, oSites, Nsimd, {...})` expands to `thread_for(sf, oSites, {...})` — one thread per osite. `acceleratorSIMTlane(Nsimd)` always returns **0** on CPU. If you need to iterate all Nsimd lanes (e.g. to extract SIMD-packed data), you must loop explicitly on CPU.
+
+On GPU, `accelerator_for` launches one SIMT thread per (osite × lane). `acceleratorSIMTlane(Nsimd)` returns the actual lane index [0, Nsimd).
+
+## Correct pattern (from MomentumProject::ImportVector)
+
+```cpp
+accelerator_for(sf, osites, Nsimd, {
+#ifdef GRID_SIMT
+  {
+    int lane = acceleratorSIMTlane(Nsimd);
+#else
+    for (int lane = 0; lane < Nsimd; lane++) {
+#endif
+      // body using lane
+    }
+  });
+```
+
+- On GPU: `GRID_SIMT` is defined → single-lane body, lane from hardware
+- On CPU: `GRID_SIMT` is not defined → explicit lane loop inside the osite thread
+
+## When is this needed?
+
+Only when you explicitly need the lane index, e.g.:
+- Extracting scalar data from SIMD-packed `vobj` via `extractLane(lane, src[sf])`
+- Computing full local coordinates from (osite, lane) → `Lexicographic::CoorFromIndex(icoor, lane, simd_layout)`
+
+When using `coalescedRead`/`coalescedWrite`, this pattern is **not needed** — those handle lane selection transparently.
+
+## Pitfall that caused a bug
+
+`A2ASpatialSum::PackVectors` originally used `accelerator_for` without the `#ifdef GRID_SIMT` lane loop. On CPU, only lane=0 was extracted, giving wrong norms (~8× too small for `GEN_SIMD_WIDTH=64`, `Nsimd=4`). Fix: add the `#ifdef GRID_SIMT` pattern. See [[ref_accelerator_for]].
+
+## Model file
+`Grid/algorithms/blas/MomentumProject.h`, function `ImportVector`, lines ~166-207.