ui: ProblemCard/ConcludeCard redesigned — the solution is the hero

Owner round (2026-10-05), Slice C:

- ProblemCard: ALWAYS OPEN — the answered card's container + eyebrow
  discipline ('Problem' label, lh-prose statement), no chevron, no
  collapsed bar. The dock mirrors it in Slice D.
- ConcludeCard: the reveal is the modelSolution committed at POSE time,
  shown verbatim as the hero — never re-authored (the slim schema's
  law). The verdict rides below in the quiz-explanation register
  (small, muted 'Verdict:' row). Nothing collapsible. Legacy inputs
  with their own solutionReveal fall back to it.
- The reveal join: modelSolutionForConclude (native-messages.ts) — the
  newest cleanly-posed problem before the conclude call; refused poses
  reveal nothing. projectView enriches tool-conclude parts with
  part.reveal at projection; ArtifactPartProps carries the field
  (contracts/chat.ts).

Pinned: always-open problem card; hero-first conclude layout with the
verdict after it; legacy fallback; the join (clean pose, refused pose,
multi-problem records) and the view enrichment.

src/core/contracts/chat.ts

@@ -270,5 +270,9 @@ export interface ArtifactPartProps {
 		toolCallId: string;
 		input?: unknown;
 		output?: unknown;
+		/** The view-seam join for conclude cards (owner round, 2026-10-05):
+		 *  the modelSolution committed at POSE time, folded onto the part by
+		 *  projectView — the reveal is never re-authored. */
+		reveal?: unknown;
 	};
 }