sccl: drive allreduce tests via torch.distributed; reorganize into tests/sccl/

Convert the multidevice allreduce correctness + latency/buffer-kind sweeps
to run through the real PyTorch-distributed path
(init_process_group(backend="ahbm") -> mp.spawn -> dist.all_reduce) instead
of direct ctx.launch, and reorganize the CCL/allreduce tests into a
tests/sccl/ package split one test per file.

Production change (required for the distributed path on non-square SIP grids):
- AhbmCCLBackend now reads explicit system.sips.w/h from the spec, with a
  square-only sqrt fallback that raises on ambiguity, instead of silently
  guessing round(sqrt(count)). This fixes the 2x3 / 3x2 torus + mesh cases,
  which previously resolved to a wrong 2x2 grid. Mirrors the test helper's
  _sip_topo_dims precedence (explicit w/h > square fallback > raise).

Test reorganization (tests/sccl/):
- _allreduce_helpers.py: shared plumbing (distributed driver, config writers,
  direct-launch run_allreduce parity reference, sweep/buffer-kind constants,
  plot aggregators, topology-diagram + FSIM-comparison emitters).
- test_allreduce_ring_torus_mesh.py: correctness across ring/torus/mesh.
- test_distributed_default_topology.py: full distributed path on topology.yaml.
- test_plot_latency_sweep.py / test_plot_buffer_kind_sweep.py: sweep rows.
- test_plot_topology_diagram.py / test_plot_comparison_fsim.py: plot emitters.
- test_intercube_root_center.py: moved in (ADR-0032 center-root latency guard).

Also:
- Move the FSIM comparison plot generator out of scripts/ into the sccl suite.
- Delete superseded test files (test_allreduce_multidevice,
  test_distributed_lrab_hierarchical_allreduce, test_allreduce_buffer_kind_sweep)
  and repoint conftest aggregators + the ipcq buffer-kind importers.
- Regenerate the allreduce_latency_plots derived artifacts from the full sweep.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

docs/diagrams/allreduce_latency_plots/summary.csv

@@ -1,37 +1,37 @@
 algorithm,sip_topology,n_sips,n_elem,bytes_per_pe,bytes_per_sip,latency_ns
-intercube_allreduce,mesh_2d_no_wrap,6,8,16,256,2666.5524999999725
-intercube_allreduce,mesh_2d_no_wrap,6,32,64,1024,2747.7399999999725
-intercube_allreduce,mesh_2d_no_wrap,6,64,128,2048,2855.98999999998
-intercube_allreduce,mesh_2d_no_wrap,6,128,256,4096,3072.4899999999725
-intercube_allreduce,mesh_2d_no_wrap,6,512,1024,16384,3336.579999999951
-intercube_allreduce,mesh_2d_no_wrap,6,1024,2048,32768,3707.49999999992
-intercube_allreduce,mesh_2d_no_wrap,6,2048,4096,65536,4449.339999999875
-intercube_allreduce,mesh_2d_no_wrap,6,4096,8192,131072,5933.020000000055
-intercube_allreduce,mesh_2d_no_wrap,6,8192,16384,262144,8900.380000000157
-intercube_allreduce,mesh_2d_no_wrap,6,16384,32768,524288,14835.099999997583
-intercube_allreduce,mesh_2d_no_wrap,6,32768,65536,1048576,26704.540000017492
-intercube_allreduce,mesh_2d_no_wrap,6,49152,98304,1572864,38573.980000026335
-intercube_allreduce,ring_1d,6,8,16,256,2365.2558333333036
-intercube_allreduce,ring_1d,6,32,64,1024,2436.9433333333036
-intercube_allreduce,ring_1d,6,64,128,2048,2532.526666666643
-intercube_allreduce,ring_1d,6,128,256,4096,2723.6933333333036
-intercube_allreduce,ring_1d,6,512,1024,16384,3042.0349999999544
-intercube_allreduce,ring_1d,6,1024,2048,32768,3390.201666666597
-intercube_allreduce,ring_1d,6,2048,4096,65536,4079.7349999998714
-intercube_allreduce,ring_1d,6,4096,8192,131072,5458.801666666721
-intercube_allreduce,ring_1d,6,8192,16384,262144,8216.93500000014
-intercube_allreduce,ring_1d,6,16384,32768,524288,13733.201666664638
-intercube_allreduce,ring_1d,6,32768,65536,1048576,24765.735000014545
-intercube_allreduce,ring_1d,6,49152,98304,1572864,35798.268333355256
-intercube_allreduce,torus_2d,6,8,16,256,1700.6024999999754
-intercube_allreduce,torus_2d,6,32,64,1024,1753.2899999999754
-intercube_allreduce,torus_2d,6,64,128,2048,1823.539999999979
-intercube_allreduce,torus_2d,6,128,256,4096,1964.0399999999754
-intercube_allreduce,torus_2d,6,512,1024,16384,2196.2849999999653
-intercube_allreduce,torus_2d,6,1024,2048,32768,2476.74499999995
-intercube_allreduce,torus_2d,6,2048,4096,65536,3037.664999999919
-intercube_allreduce,torus_2d,6,4096,8192,131072,4159.50500000003
-intercube_allreduce,torus_2d,6,8192,16384,262144,6403.185000000081
-intercube_allreduce,torus_2d,6,16384,32768,524288,10890.544999998769
-intercube_allreduce,torus_2d,6,32768,65536,1048576,19865.265000008738
-intercube_allreduce,torus_2d,6,49152,98304,1572864,28839.985000013185
+lrab_hierarchical_allreduce,mesh_2d_no_wrap,6,8,16,256,2666.552500000015
+lrab_hierarchical_allreduce,mesh_2d_no_wrap,6,32,64,1024,2747.7400000000152
+lrab_hierarchical_allreduce,mesh_2d_no_wrap,6,64,128,2048,2855.990000000018
+lrab_hierarchical_allreduce,mesh_2d_no_wrap,6,128,256,4096,3072.490000000019
+lrab_hierarchical_allreduce,mesh_2d_no_wrap,6,512,1024,16384,3337.1133333333582
+lrab_hierarchical_allreduce,mesh_2d_no_wrap,6,1024,2048,32768,3708.0333333333692
+lrab_hierarchical_allreduce,mesh_2d_no_wrap,6,2048,4096,65536,4449.873333333393
+lrab_hierarchical_allreduce,mesh_2d_no_wrap,6,4096,8192,131072,5933.020000000124
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+lrab_hierarchical_allreduce,mesh_2d_no_wrap,6,32768,65536,1048576,26704.540000000765
+lrab_hierarchical_allreduce,mesh_2d_no_wrap,6,49152,98304,1572864,38573.97999999701
+lrab_hierarchical_allreduce,ring_1d,6,8,16,256,2365.255833333347
+lrab_hierarchical_allreduce,ring_1d,6,32,64,1024,2436.9433333333473
+lrab_hierarchical_allreduce,ring_1d,6,64,128,2048,2532.526666666683
+lrab_hierarchical_allreduce,ring_1d,6,128,256,4096,2723.693333333349
+lrab_hierarchical_allreduce,ring_1d,6,512,1024,16384,3048.635000000021
+lrab_hierarchical_allreduce,ring_1d,6,1024,2048,32768,3393.4016666666957
+lrab_hierarchical_allreduce,ring_1d,6,2048,4096,65536,4082.401666666714
+lrab_hierarchical_allreduce,ring_1d,6,4096,8192,131072,5458.80166666677
+lrab_hierarchical_allreduce,ring_1d,6,8192,16384,262144,8216.934999999943
+lrab_hierarchical_allreduce,ring_1d,6,16384,32768,524288,13733.201666665835
+lrab_hierarchical_allreduce,ring_1d,6,32768,65536,1048576,24765.73500000064
+lrab_hierarchical_allreduce,ring_1d,6,49152,98304,1572864,35798.268333331536
+lrab_hierarchical_allreduce,torus_2d,6,8,16,256,1700.6025000000095
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+lrab_hierarchical_allreduce,torus_2d,6,32768,65536,1048576,19865.265000000378
+lrab_hierarchical_allreduce,torus_2d,6,49152,98304,1572864,28839.98500000059