skills: add GPU/A2A reference skill documents

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

skills/ref_a2a_emf_work.md

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+---
+name: ref_a2a_emf_work
+description: "A2A Extended Meson Field GPU offload work — status, file locations, pending task"
+metadata: 
+  node_type: memory
+  type: project
+  originSessionId: 956e80aa-401d-481a-80bb-17f8abe1c131
+---
+
+## What was built
+
+`Grid/algorithms/blas/A2ASpatialSum.h` — batched GEMM spatial sum replacing scalar SIMD accumulation. Included via `Grid/algorithms/Algorithms.h`.
+
+`tests/Test_extended_meson_field.cc` — test with class `A2AExtendedMesonFieldRef` containing:
+- CPU reference path (`use_blas=false`)
+- BLAS path (`use_blas=true`) using `A2ASpatialSum`
+- Per-phase timing with `[ref type=N]` / `[blas type=N]` labels
+- 4 contraction types (0-3), all verified at machine precision (~4e-16 rel_err)
+
+## Pending task: GPU offload class
+
+**Goal**: Write `A2AExtendedMesonFieldGPU` in the same test file, replacing all `thread_for` loops with `accelerator_for`-based free function kernels.
+
+The `thread_for` blocks to replace all have the form:
+```cpp
+thread_for(r, rd, {
+  int so = r * grid->_ostride[orthogdim];
+  for (int n = 0; n < e1; n++)
+  for (int b = 0; b < e2; b++) {
+    int ss = so + n * stride + b;
+    // work
+  }
+});
+```
+Replace with `accelerator_for(ss, grid->oSites(), Nsimd, { ... })`.
+
+**Free functions to write** (each takes `Lattice<T>` args, opens views internally):
+- `A2ALoopPropagator` — outerProduct sum (loop build)
+- `A2APackLeftConjugated` — conjugate left fermion fields into `Lattice<SpinColourVector_v>`
+- `A2ALoopLeftContractionType0/1/2/3` — per-site loop × loop propagator → `tloop`
+- `A2ALoopRightContractionType0/1/2/3` — per-site tloop × right → `loopRight[j]`
+
+**Data structure changes required**:
+- `tloopv`: `std::vector<SpinColourMatrix_v>` → `Lattice<SpinColourMatrix_v>` (PropagatorField)
+- `leftv[i]`: `std::vector<SpinColourVector_v>` → `Lattice<SpinColourVector_v>`
+- `loopRight[j]`: `std::vector<SpinColourVector_v>` → `Lattice<SpinColourVector_v>`
+
+**Why**: `std::vector<vobj>` is host memory, not GPU accessible. See [[ref_lattice_vs_vector]].
+
+**`A2ASpatialSum` impact**: `PackLeft`/`PackRight` currently take `std::vector<std::vector<vobj>>`. Once leftv/loopRight become `std::vector<Lattice<vobj>>`, those signatures must change to match.
+
+## Timing on 8.8.8.16 (N_i=N_j=8, Nloop=4, 1 MPI rank)
+
+Dominant costs:
+- `loop_build`: 4-6 ms (outerProduct over 4 propagators)
+- `pack_loopright`: 0.9-2.2 ms (type-dependent)
+- `spatial_sum` (ref): ~1.5 ms
+- `A2ASpatialSum TOTAL`: 2.5-4.3 ms (PackLeft+PackRight dominate GEMM on small volume)
+
+## Related
+[[ref_accelerator_for]] [[ref_coalesced_views]] [[ref_lattice_vs_vector]] [[ref_grid_simt_pattern]]