exp(06): focused investigation — visual hierarchy without layout changes

Emphasise the active investigation card through stronger elevation,
clearer borders, and improved spacing. Quiet supporting panels by
reducing border opacity, softening heading weight, and lowering
text contrast — making them available without competing for attention.

Facilitator card receives a warm surface tint to read as a briefing
card rather than a generic panel.

Documentation: close experiment 05 with findings, add experiment 06
to the design evolution log, add Attention Hierarchy to UX guidelines,
defer dark mode to a future Investigation Mode experiment.

Presentation changes only — no reasoning, prompts, graph, API, or
backend modifications.

docs/design-evolution-log.md

@@ -293,6 +293,33 @@ Questions
 - Does "Don't show again" feel preferable to automatically hiding the introduction?
 - Should the facilitator panel become an optional workspace companion rather than mandatory onboarding?
 
+Status:
+Completed.
+
+Findings:
+
+- A horizontal facilitator/workspace arrangement feels more natural than stacked onboarding.
+- The workspace becomes the visual destination rather than the introduction.
+- User-controlled dismissal is preferable to automatic hiding.
+- The facilitator feels useful but visually too passive.
+- Remaining issues are now visual hierarchy rather than layout architecture.
+
+### Experiment 06 — Focused Investigation
+
+Hypothesis
+
+The interface should gently guide attention towards the current task without hiding supporting information.
+
+Reducing competition between panels may improve concentration more than introducing additional colour or decoration.
+
+Questions
+
+- Does visual emphasis naturally guide the eye?
+- Can supporting panels become quieter without disappearing?
+- Does the investigation question become the obvious focal point?
+- Does the workspace feel calmer?
+- Are we approaching a professional investigation environment?
+
 Status:
 Experimental.
 
@@ -318,3 +345,11 @@ The "Don't show this introduction again" checkbox uses sessionStorage as a place
 This preference should eventually be handled through user preferences or settings rather than local component state.
 
 TODO: When user accounts are introduced, persist this preference to the user profile so it travels across devices and sessions.
+
+## Future Note — Dark Mode
+
+Dark mode is intentionally deferred.
+
+Once the information architecture and visual hierarchy stabilise we will investigate whether an "Investigation Mode" (rather than a conventional dark mode) improves concentration.
+
+This should be treated as a future UX experiment rather than an accessibility feature.