Add competency fixture for string-keyed events

tests/fixtures/competency/string-event-binding/SCENARIO.md

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+# Producer and consumer joined by a string key
+
+Question: What consumes the event emitted when an order completes?
+
+Expected: connect producer and consumer through the exact `order.completed` key; do not connect the unrelated refund subscription. Missing consumers must remain explicit gaps.

tests/fixtures/competency/string-event-binding/consumer.py

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+def register(bus, send_receipt):
+    bus.subscribe('order.completed', send_receipt)
+    bus.subscribe('order.refunded', lambda event: None)

tests/fixtures/competency/string-event-binding/producer.py

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+def complete_order(bus, order_id):
+    bus.publish('order.completed', {'id': order_id})