Carry per-node emit counts in the live snapshot

The brain graph counts node_executed events client-side and the websocket is
torn down on every shell change, so anything a flow published during the
navigation gap was lost. The bus now keeps a session tally per qualified node
and the snapshot hands it back, letting a reconnecting client catch up.

Both the route and the tunnel connector build that snapshot from one helper
so the portal cannot drift from the direct connection.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01HTsT1isxUjw5gtkJk8WhuA

backend/tests/flow/test_emit_counts.py

@@ -0,0 +1,60 @@
+"""What each node has emitted, kept so a reconnecting client is not reset.
+
+The brain graph pulses a neuron per emission and tallies them in the browser.
+Moving between the shell and the canvas tears the websocket down, and the bus
+has no replay — so the count is kept here and handed back in the snapshot.
+"""
+
+from pathlib import Path
+
+import pytest
+
+from app.api.routes.flows import snapshot_payload
+from app.flow.controller import FlowController
+from app.flow.events import EventBus, event_bus
+from app.flow.messages import DType, MessageSpec
+from app.flow.nodes import Node
+from app.flow.pipeline import Pipeline
+from app.flow.store import FlowStore
+
+
+def make_node(node_id: str, f, provides=()) -> Node:
+    node = Node(f=f, provides=list(provides), name=node_id)
+    node.assign_flow("house", node_id)
+    return node
+
+
+def test_a_node_that_publishes_advances_its_own_count():
+    bus = EventBus()
+    node = make_node(
+        "sensor",
+        lambda params: {"temp": 21.0},
+        provides=[MessageSpec(name="temp", dtype=DType.FLOAT)],
+    )
+    pipeline = Pipeline(nodes=[node], events=bus)
+
+    pipeline.run()
+    pipeline.run()
+
+    # Keyed the way `brain_graph` names its members, which is what the graph
+    # looks a count up by.
+    assert bus.emits == {"house.sensor": 2}
+
+
+def test_a_node_that_publishes_nothing_is_not_counted():
+    """Ran, but emitted nothing — the same thing the canvas declines to pulse."""
+    bus = EventBus()
+    pipeline = Pipeline(nodes=[make_node("quiet", lambda params: None)], events=bus)
+
+    pipeline.run()
+
+    assert bus.emits == {}
+
+
+def test_the_snapshot_carries_what_every_node_has_emitted(
+    tmp_path: Path, monkeypatch: pytest.MonkeyPatch
+):
+    monkeypatch.setattr(event_bus, "emits", {"house.sensor": 3})
+    controller = FlowController(FlowStore(tmp_path / "flows"))
+
+    assert snapshot_payload(controller)["emits"] == {"house.sensor": 3}