spec: add fraud detection engine specification (001)

4 user stories: alert investigation (P1), transaction ingestion
+ rule engine (P1), rule management (P2), dashboard (P3).
16 functional requirements, 7 success criteria, 5 entities.
Quality checklist: all 16 items passed.

.specify/feature.json

@@ -0,0 +1 @@
+{"feature_directory":"specs/001-fraud-detection-engine"}