Intercube allreduce: pe0 cube-mesh reduce + multi-SIP ring/torus/mesh

New intercube allreduce kernel replacing the old flat ring algorithms.
Reduces across the 4x4 cube mesh within each SIP (pe0-only, same-lane),
then inter-SIP exchange on root cube, then broadcast back. Supports
ring_1d, torus_2d, and mesh_2d_no_wrap SIP topologies driven by
topology.yaml. Integrated with dist.init_process_group / dist.all_reduce.

New files:
- src/kernbench/ccl/algorithms/intercube_allreduce.py (kernel)
- src/kernbench/ccl/sfr_config.py (configure_sfr_intercube_multisip)
- tests/test_allreduce_multidevice.py (config-driven, 3 topologies)
- tests/test_distributed_intercube_allreduce.py (full distributed path)
- tests/test_intercube_sfr_config.py (SFR wiring verification)

Modified:
- distributed.py: AhbmCCLBackend uses configure_sfr_intercube_multisip
- topologies.py: added torus_2d, mesh_2d_no_wrap
- install.py: global_E/W/N/S in _OPPOSITE_DIR
- topology.yaml: added system.sips.topology
- ccl.yaml: single intercube_allreduce algorithm
- benches/ccl_allreduce.py: row_wise cube-mesh tensor layout

Removed old flat-ring algorithms and their tests.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/kernbench/ccl/topologies.py

@@ -73,6 +73,39 @@ def tree_binary(rank: int, world_size: int) -> NeighborMap:
     return n
 
 
+def torus_2d(rank: int, world_size: int) -> NeighborMap:
+    """Square 2D torus (N/S/E/W) with wrap-around on all edges.
+
+    Alias for mesh_2d (which already wraps). Explicit name for clarity
+    when used as a SIP-level topology.
+    """
+    return mesh_2d(rank, world_size)
+
+
+def mesh_2d_no_wrap(rank: int, world_size: int) -> NeighborMap:
+    """Square 2D mesh (N/S/E/W) WITHOUT wrap-around.
+
+    Edge nodes have fewer neighbors (no wrapping). Used for SIP-level
+    topologies where physical links don't wrap.
+    """
+    side = int(round(world_size ** 0.5))
+    if side * side != world_size:
+        raise ValueError(
+            f"mesh_2d_no_wrap requires square world_size, got {world_size}"
+        )
+    r, c = divmod(rank, side)
+    n: NeighborMap = {}
+    if r > 0:
+        n["N"] = (r - 1) * side + c
+    if r < side - 1:
+        n["S"] = (r + 1) * side + c
+    if c > 0:
+        n["W"] = r * side + (c - 1)
+    if c < side - 1:
+        n["E"] = r * side + (c + 1)
+    return n
+
+
 def none(rank: int, world_size: int) -> NeighborMap:
     """Empty map — algorithm's neighbors() must build from scratch."""
     return {}
@@ -82,6 +115,8 @@ _BUILTIN: dict[str, TopologyFn] = {
     "ring_1d": ring_1d,
     "ring_1d_unidir": ring_1d_unidir,
     "mesh_2d": mesh_2d,
+    "torus_2d": torus_2d,
+    "mesh_2d_no_wrap": mesh_2d_no_wrap,
     "tree_binary": tree_binary,
     "none": none,
 }