Complete enemy event reference documentation

docs/internal/dnd.md

@@ -102,6 +102,11 @@ relatedness validators report advisory evidence concerns. The configured order
 is documented in
 [Configuration](../config.md#production-validator-keys-and-default-chains).
 
+Enemy-event extraction additionally rejects a second `engaged` observation for
+the same comparison identity within one scene-scoped result. Normalization may
+combine results from distinct scenes, so it intentionally does not apply that
+rule. Configuration owns the exact validator key and chain position.
+
 Normalizers are deterministic for spells, combat turns, item events, NPC
 interactions, scene descriptions, and enemy events. They canonicalize display
 values and evidence, use source-document order for stable output, and issue