architecture: introduce investigation narrative layer

.claude/ux-guidelines.md

@@ -514,3 +514,22 @@ Three tiers, applied top to bottom:
 - Prefer labels over descriptions when labels are more concise and clear.
 - Omit items too verbose to scan; do not synthesise rewritten claims.
 - Never invent facts absent from the graph.
+
+## Investigation Narrative
+
+The reasoning graph is the machine representation of the investigation.
+
+The investigation narrative is the human representation.
+
+The UI renders projections from the narrative, not directly from the graph.
+
+Principles:
+
+- Users understand investigations, not graphs.
+- The graph is an internal reasoning structure.
+- The narrative is the explanation of current understanding.
+- Every user-facing panel should consume narrative state where possible.
+- Multiple UI layouts may share the same narrative.
+- Narrative should evolve as evidence changes.
+- Narrative must never invent facts absent from the graph.
+- Narrative explains uncertainty rather than exposing graph mechanics.