fix(coins): rebuild machines array when cached .npz has wrong length

Stale machines_*.npz files on disk can have a different length than the
current Leiden partition's num_communities, causing an IndexError during
COINs experiment preparation. Rebuild machines = zeros(num_communities+2)
in both _load_all_loaders and the cached-loader path in
_load_coins_experiment to match the website's single-device setup.

src/backend/api/services/registry.py

@@ -467,6 +467,17 @@ class ModelRegistry:
                         seed=seed, device="cpu", val_size=0.01, test_size=0.02,
                         community_method="leiden", leiden_resolution=leiden_resolution,
                     )
+                    # Rebuild machines from the current num_communities: cached machines_*.npz
+                    # files on disk can be stale relative to the current subgraphing output
+                    # (different num_communities), and the website always runs num_machines=1.
+                    import numpy as np
+                    expected_len = loader.num_communities + 2
+                    if len(loader.machines) != expected_len:
+                        logger.warning(
+                            "Stale machines.npz for %s: len=%d but num_communities+2=%d; rebuilding",
+                            dataset_id, len(loader.machines), expected_len,
+                        )
+                    loader.machines = np.zeros(expected_len, dtype=int)
                     self.loaders[dataset_id] = loader
                     # Share this loader with _load_coins_experiment so experiments for the
                     # same (dataset, seed, leiden_resolution) reuse it instead of reloading
@@ -704,6 +715,13 @@ class ModelRegistry:
             loader_key = (dataset_id, seed, leiden_resolution)
             if loader_key in self._coins_loaders:
                 cached_loader = self._coins_loaders[loader_key]
+                # Defensive: ensure machines length matches current num_communities.
+                # _load_all_loaders already does this, but any future code path that
+                # populates _coins_loaders directly could skip it.
+                import numpy as np
+                expected_len = cached_loader.num_communities + 2
+                if len(cached_loader.machines) != expected_len:
+                    cached_loader.machines = np.zeros(expected_len, dtype=int)
                 experiment.loader = cached_loader
                 # Temporarily replace load_graph with a no-op: prepare() will find all
                 # required attributes (num_nodes, communities, graph_indexes, …) already set.