fix(registry): store nodes per-client to prevent multi-tab race condition

Move NodeRegistry from a single global _nodes dict to a per-client
(_tab_nodes) structure so that multiple ComfyUI browser tabs no
longer overwrite each other's workflow node data during a
lora_registry_refresh cycle.  The merged result is a union of all
known tabs' target nodes, eliminating the non-deterministic failure
where send-to-workflow could randomly target a tab lacking valid
targets.

- NodeRegistry.register_nodes(sid, nodes) replaces per-tab data
  without affecting other tabs.
- NodeRegistry.get_merged_registry() returns the union across all
  connected clients, together with tab_count / per-tab metadata.
- prepare_for_refresh() snapshots the current active sockets; caller
  re-reads before merging so that newly-connected tabs are not pruned.
- workflow_registry.js sends api.clientId in the POST body so the
  backend can identify which tab is registering.

tests/routes/test_api_snapshots.py

@@ -60,7 +60,9 @@ class FakePromptServer:
     sent = []
 
     class Instance:
-        def send_sync(self, event, payload):
+        sockets: dict = {}
+
+        def send_sync(self, event, payload, sid=None):
             FakePromptServer.sent.append((event, payload))
 
     instance = Instance()
@@ -148,7 +150,8 @@ class TestNodeRegistryHandlerSnapshots:
                         "type": "Lora Loader (LoraManager)",
                         "title": "Test Loader",
                     }
-                ]
+                ],
+                "client_id": "test-client-1",
             }
         )
 
@@ -167,7 +170,7 @@ class TestNodeRegistryHandlerSnapshots:
             standalone_mode=False,
         )
 
-        request = FakeRequest(json_data={"nodes": []})
+        request = FakeRequest(json_data={"nodes": [], "client_id": "test-client-1"})
         response = await handler.register_nodes(request)
         payload = json.loads(response.text)