Constant INSERT_SIGNAL_EVENT_SQL 

pub const INSERT_SIGNAL_EVENT_SQL: &str = "INSERT INTO ff_signal_event \
     (execution_id, signal_id, waitpoint_id, source_identity, \
      delivered_at_ms, partition_key, namespace, instance_tag) \
     SELECT ?1, ?2, ?3, ?4, ?5, ?6, \
            json_extract(raw_fields, '$.namespace'), \
            json_extract(raw_fields, '$.tags.\"cairn.instance_id\"') \
       FROM ff_exec_core \
      WHERE partition_key = ?6 AND execution_id = ?7 \
     UNION ALL \
     SELECT ?1, ?2, ?3, ?4, ?5, ?6, NULL, NULL \
      WHERE NOT EXISTS ( \
          SELECT 1 FROM ff_exec_core \
           WHERE partition_key = ?6 AND execution_id = ?7 \
      )";

Append one signal-delivery event to the outbox.

Binds:

• 1 = execution_uuid stringified — ff_signal_event.execution_id is TEXT (mirrors PG at ff-backend-postgres/migrations/0007).
• 2 = signal_id stringified — TEXT column.
• 3 = waitpoint_id stringified (nullable) — TEXT column.
• 4 = source_identity (nullable) — TEXT.
• 5 = delivered_at_ms.
• 6 = partition_key (reused in the WHERE clause).
• 7 = execution_uuid as BLOB — ff_exec_core.execution_id is stored as a 16-byte BLOB; the co-transactional SELECT looks it up by BLOB equality while emitting the TEXT stringification onto ff_signal_event.

The SQLite ff_signal_event table populates namespace + instance_tag from ff_exec_core.raw_fields via a co-transactional SELECT — same pattern as the PG reference (ff-backend-postgres/src/signal_event.rs:33-49). json_extract is the SQLite JSON1 analogue of PG’s ->> operator.