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/facilitator-behaviour.md

@@ -6,6 +6,22 @@ This document records observed patterns from years of expert consulting practice
 
 ---
 
+## Behaviour Selection
+
+Facilitator behaviours are selected from Investigation State Assessment.
+
+Behaviours do not inspect graph nodes directly.
+
+Behaviours consume assessment.
+
+This keeps behaviour independent from reasoning implementation.
+
+The assessment layer (introduced in Experiment 16) describes the investigation's current state across multiple analytical dimensions: phase, progress, evidence quality, understanding trajectory, uncertainty trend, conversation health, and behaviour readiness. Behaviour selection operates on this description — not on raw graph structure.
+
+The decision of *which* behaviour to deploy is a separate concern from the inventory of *what* behaviours exist. This document defines the latter. The former depends on the assessment layer described in `docs/investigation-state-assessment.md`.
+
+---
+
 ## What Experiment 14 proved
 
 Experiment 14 proved architecturally that: