exp(18): implement investigation state assessment layer

Implement the three-dimensional assessment (phase, progress, conversation
health) that sits between narrative and behaviour selection.

Key changes:
- lib/assessment/investigation-state-assessor.js: assessor module with
  countObservations, assessPhase, assessProgress, assessConversationHealth,
  assessInvestigationState — deterministic classifiers using known rules
- tests/investigation-state-assessor.test.js: 51 tests covering phase
  classification (orienting→concluding), progress thresholds, health
  conditions, confidence aggregation, edge cases, and observation counting
- lib/graph/orchestrator.js: integration calls passing correctly-shaped input
  to assessInvestigationState() at three call sites (~552, ~904, ~1013)

Design decisions encoded in this iteration:
- countObservations counts nodes with known/resolved status + high-confidence
  non-unknown non-state nodes (not just explicit observation-kind nodes)
- Phase uses seven values including cannot_determine for insufficient data
- Progress uses resolution ratio thresholds: accelerating (>0.6), steady
  (0.2-0.6), stalled (<0.2 with ≥1 resolved)
- Overall confidence = minimum across all three dimensions (conservative)

Also adds investigation-state-assessment-contract.md and updates
design-evolution-log, investigation-state-assessment.md (status header),
and investigation-turn-cycle.md (implementation status table).

lib/graph/orchestrator.js

@@ -18,6 +18,7 @@ import {
   determineGraphBackedQuestion,
 } from "./apply-proposal.js";
 import { buildGraphUpdatePrompt } from "./prompt-builder.js";
+import assessInvestigationState from "../assessment/investigation-state-assessor.js";
 import {
   buildReasoningState,
   formulateQuestion,
@@ -548,6 +549,20 @@ export async function startCase(body) {
         initialQuestionResult.graphReasoningIntegrity ?? null,
       noQuestionReason: initialQuestionResult.noQuestionReason ?? null,
     }),
+    assessment: assessInvestigationState({
+      situationGraph,
+      selectedQuestion,
+      noQuestionReason: initialQuestionResult.noQuestionReason ?? null,
+      diagnostics: {
+        promptVersion,
+        modelName: analysis?.modelName ?? null,
+        responseDurationMs: analysis?.responseDurationMs ?? null,
+        validationStatus: analysis?.validationStatus ?? "invalid",
+        nodeCount: situationGraph?.nodes?.length ?? 0,
+        edgeCount: situationGraph?.edges?.length ?? 0,
+        reasoningPattern: initialQuestionResult.selectedQuestion?.reasoningPattern ?? null,
+      },
+    }),
   };
 }
 
@@ -886,8 +901,22 @@ async function updateCaseWithDependencies(body, dependencies = {}) {
           applicationResult.selectedQuestion,
         ),
       }),
-    };
-  }
+    assessment: assessInvestigationState({
+      situationGraph: applicationResult.updatedSituationGraph,
+      selectedQuestion: applicationResult.selectedQuestion,
+      noQuestionReason: applicationResult.noQuestionReason ?? null,
+      diagnostics: {
+        promptVersion,
+        modelName,
+        responseDurationMs,
+        validationStatus: "valid",
+        nodeCount: applicationResult.updatedSituationGraph?.nodes?.length ?? 0,
+        edgeCount: applicationResult.updatedSituationGraph?.edges?.length ?? 0,
+        reasoningPattern: applicationResult.selectedQuestion?.reasoningPattern ?? null,
+      },
+    }),
+  };
+}
 
   return {
     success: true,
@@ -981,5 +1010,19 @@ async function updateCaseWithDependencies(body, dependencies = {}) {
         null,
       ),
     }),
+    assessment: assessInvestigationState({
+      situationGraph,
+      selectedQuestion: null,
+      noQuestionReason: null,
+      diagnostics: {
+        promptVersion,
+        modelName,
+        responseDurationMs,
+        validationStatus: "valid",
+        nodeCount: situationGraph?.nodes?.length ?? 0,
+        edgeCount: situationGraph?.edges?.length ?? 0,
+        reasoningPattern: null,
+      },
+    }),
   };
 }