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).

docs/investigation-state-assessment.md

@@ -1,6 +1,7 @@
 # Investigation State Assessment — Architectural Specification
 
-> This is a design document only. Do not implement yet.
+> **Status: Implemented (Experiment 18). First executable slice deployed.**
+> Design evolved through experiments; implementation validates and adjusts the spec iteratively.
 
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