architecture: introduce investigation state assessment

Close Experiment 15 (Facilitator Behaviour Specification).

Introduce Experiment 16 — Investigation State Assessment.

- Create docs/investigation-state-assessment.md with 7 assessment dimensions:
  Current Investigation Phase, Investigation Progress, Evidence Quality,
  Understanding Trajectory, Uncertainty Trend, Conversation Health,
  and Behaviour Readiness. Each dimension includes purpose, observable
  signals, possible values, and how behaviours may consume it.

- Document 6 assessment principles (Assess Not Decide, All Signals
  Traceable to Narrative, Descriptive Not Prescriptive, Convergence Over
  Single Signal, Stateful Across Turns, Uncertainty About Assessment Is
  Itself Assessable).

- Include exploratory decision matrix linking investigation states to
  likely behaviours with reasons.

- Prepend Behaviour Selection section to docs/facilitator-behaviour.md
  recording that behaviours are selected from Investigation State
  Assessment and do not inspect graph nodes directly.

- Update docs/design-evolution-log.md: close Experiment 15, add
  Experiment 16 closure, record emerging architecture with the new layer
  between Narrative and Behaviour Selection.

No implementation. Documentation only. No changes to reasoning engine,
graph generation, prompts, orchestrator, APIs, Ollama integration, or UI.

docs/design-evolution-log.md

@@ -784,6 +784,91 @@ Confirmed as the correct next direction.
 
 Commit the behavioural specification. Do not implement yet. Future experiments will integrate behavioural assessment into the reasoning cycle. This document defines what the facilitator does; future work determines how the system implements it.
 
+**Status: Closed.** The behavioural model is established and documented. The gap it identified — that behaviours need a decision process operating on investigation state rather than graph structure — becomes the focus of Experiment 16.
+
+---
+
+### Experiment 16 — Investigation State Assessment
+
+#### Hypothesis
+
+The facilitator should never inspect the graph directly when deciding what to do next.
+
+Instead it should act upon an assessment of the investigation — its phase, progress, evidence quality, understanding trajectory, uncertainty trend, conversation health, and behaviour readiness.
+
+This assessment is distinct from both:
+
+- The reasoning graph (which captures *what* is known)
+- The investigation narrative (which translates *what is known* into human language)
+
+The assessment answers: *Given where we are, what kind of help is most appropriate right now?*
+
+No reasoning changes.
+
+No prompt changes.
+
+No UI changes.
+
+This is an architectural experiment.
+
+#### Status
+
+Architectural.
+
+#### Evaluation
+
+Pending.
+
+---
+
+### Experiment 16 — Emerging Architecture Observation
+
+The Confidence Engine architecture is becoming:
+
+User
+
+↓
+
+Facilitated Conversation (where behaviour lives)
+
+↓
+
+Behaviour Selection (consumes assessment output)
+
+↓
+
+Investigation State Assessment (describes investigation)
+
+↓
+
+Investigation Narrative (human representation of state)
+
+↓
+
+Reasoning Graph (machine representation)
+
+↓
+
+LLM / Ollama / Reasoning Engine
+
+↓
+
+User
+
+This is not a final design. It is an observation emerging from 16 experiments.
+
+What is becoming clear:
+
+- The reasoning graph is the machine representation.
+- The investigation narrative is the human representation.
+- The investigation state assessment is the decision representation — it translates state into readiness signals for behaviour selection.
+- Behaviour selection determines what kind of help to deploy.
+- Facilitated Conversation is where that help is delivered.
+
+Each layer has a single responsibility. Each feeds the next. No layer inspects another's implementation details.
+
+This architecture emerged from observation, not top-down design. It may still change as future experiments test it.
+
 ---
 
 ## Emerging Direction — Graph as Source of Truth