feat: explore facilitated investigation workspace

Phase 4 UX exploration — workshop desk metaphor.

Workspace layout changes:
- Investigation Map promoted from preview to workspace artefact
- Understanding card given wider surface (lg:col-span-2)
- Grid shifts from equal-column to cognitive-weighted widths (lg:grid-cols-5)
- Mobile remains stacked; tablet simplifies naturally
- Desktop exploits wider working canvas

Layout structured by cognitive activity:
  Active workspace zone (Question + Response)
  Supporting workspace (Understanding, Map)
  Reference row (Situation, History)

docs/design-evolution-log.md

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+# Design Evolution Log
+
+A chronological record of why significant design decisions were made. This is NOT a changelog. It records the product's evolution of thinking.
+
+---
+
+## Phase 1
+Simple conversational investigation
+
+Question → Answer interaction.
+
+Purpose:
+Prove the reasoning loop.
+
+Learning:
+Conversation alone does not provide sufficient context during longer investigations.
+
+---
+
+## Phase 2
+Persistent investigation notebook
+
+Added:
+
+- current understanding
+- original situation
+- investigation history
+
+Learning:
+Users need persistent context rather than remembering previous answers.
+
+---
+
+## Phase 3
+Document workspace
+
+Created a coherent workspace with:
+
+- investigation status
+- current investigation
+- response
+- understanding
+- investigation map placeholder
+- situation
+- history
+
+Learning:
+The interface became usable but still behaved like a document rather than a workspace.
+
+---
+
+## Phase 4 (Current Exploration)
+Facilitated Investigation Workshop
+
+Status:
+Experimental.
+
+Hypothesis:
+
+The Confidence Engine is not:
+
+- a chatbot
+- a dashboard
+- a form
+
+It is a facilitated investigation workspace.
+
+The interface should resemble the environment in which structured thinking happens.
+
+Record discoveries rather than conclusions.
+
+Leave room for future phases.
+
+---
+
+## Phase 4 — Guiding Principles
+
+The Confidence Engine is a workspace, not a document.
+
+People think in multiple directions simultaneously.
+
+Useful context should be visible together.
+
+The interface should favour thinking over scrolling.
+
+The workspace should feel like a large desk or workshop rather than a narrow report.
+
+The engine facilitates thinking.
+
+The user contributes evidence.
+
+The workspace captures shared understanding.