feat(confidence-engine): render INVESTIGATING cue on Open Question cards with focused history

- ThreadContributionsBadge (rendered per-node on OpenQuestionsPanel
  cards and Done-for-now cards) now shows an amber INVESTIGATING
  indicator when the node has matching contributions via targetNodeId
  identity match.
- UNCLEAR and INVESTIGATING cues coexist independently on the same
  card — UNCLEAR is epistemic state, INVESTIGATING is activity cue.
- Deterministic test suite added: focused-investigation-history (11
  tests) covering identity matching, zero-contrib edge cases,
  multiple-contrib coalescing, done-for-now retention, and uncoupling
  from UNCLEAR state.
- All 57 tests pass.

components/reasoning-workspace.jsx

@@ -581,6 +581,10 @@ function ThreadContributionsBadge({ nodeId, contributions }) {
 
   return (
     <div className="mt-3">
+      {/* Thread activity cue: visible when this node has focused investigation history */}
+      <span className="mb-1 block text-[9px] uppercase tracking-widest font-semibold text-amber-500/70">
+        INVESTIGATING
+      </span>
       {/* Thread learning indicator — collapsed by default; user can expand to inspect history */}
       <details open={false} className="rounded-lg border border-gray-200/80 bg-white/60">
         <summary className="cursor-pointer px-3 py-1.5 text-xs font-medium text-gray-600 hover:text-gray-800 select-none">