Enforce min_nodes >= 2 for RDMA (MlxJaccl) instances

RDMA requires at least 2 nodes — a single-node RDMA instance is
nonsensical. Enforce this in both the dashboard (when building the
launch request) and the backend placement (when filtering cycles).
Previously, selecting RDMA would still place on 1 node because
min_nodes defaulted to 1 and the placement silently switched to Ring.

Co-Authored-By: Claude Opus 4.6 <[email protected]>

dashboard/src/routes/+page.svelte

@@ -643,7 +643,12 @@
           model_id: modelId,
           sharding: preview?.sharding ?? selectedSharding,
           instance_meta: preview?.instance_meta ?? selectedInstanceType,
-          min_nodes: Math.max(selectedMinNodes, nodeFilter.size),
+          min_nodes: Math.max(
+            selectedMinNodes,
+            nodeFilter.size,
+            // RDMA requires at least 2 nodes
+            (preview?.instance_meta ?? selectedInstanceType) === "MlxJaccl" ? 2 : 1,
+          ),
           node_ids: nodeFilter.size > 0 ? Array.from(nodeFilter) : undefined,
         }),
       });

src/exo/master/placement.py

@@ -63,7 +63,14 @@ def place_instance(
     required_nodes: set[NodeId] | None = None,
 ) -> dict[InstanceId, Instance]:
     cycles = topology.get_cycles()
-    candidate_cycles = list(filter(lambda it: len(it) >= command.min_nodes, cycles))
+    # RDMA (MlxJaccl) requires at least 2 nodes
+    effective_min_nodes = max(
+        command.min_nodes,
+        2 if command.instance_meta == InstanceMeta.MlxJaccl else 1,
+    )
+    candidate_cycles = list(
+        filter(lambda it: len(it) >= effective_min_nodes, cycles)
+    )
 
     # Filter to cycles containing all required nodes (subset matching)
     if required_nodes: