feat: classify observation relationships after comparability

docs/v0.6-comparability-experiment.md

@@ -17,8 +17,19 @@ The engine should confirm that observations are comparable before treating their
 
 - The first four scenarios repeated the same failure pattern: contradiction-level investigation could begin before comparability was established.
 - A deterministic comparability gate corrected that by producing one comparison question first.
-- Temperature increased / Ice melted was treated as comparability confirmed, so no comparison question was asked.
-- Sales doubled / Sales doubled was treated as comparability confirmed, and contradiction reasoning was not needed.
+- Confirmed comparability did not by itself imply contradiction.
+- Temperature increased / Ice melted was reclassified as a compatible relationship, so no contradiction question was asked.
+- Sales doubled / Sales doubled was reclassified as duplicate observations, so no follow-up question was asked.
+
+## Relationship classification stage
+
+After comparability assessment, observations now pass through a deterministic relationship classification stage:
+
+- `contradictory`
+- `compatible`
+- `potentially_related`
+- `duplicate`
+- `insufficient_information`
 
 ## Whether comparability should become a permanent reasoning stage