practice UI: the dock, the transcript cards, layered-Esc discard

- PracticeDock (components/practice-dock.tsx): the active problem
  replaces the composer — statement (markdown+LaTeX) above, the 'Your
  thinking' composer-twin below with the two submit lanes (Nudge =
  thinking without a commit; Commit = answer commit; ⌘/Ctrl+Enter
  commits). Dictation claims the thinking field.
- Transcript cards (practice-cards.tsx + the artifact registry): the
  problem card (compact-expandable statement — the commitment NEVER
  renders), the conclude card (outcome + verdict + reveal; the
  dismissal synthesis is one muted line), and the user-side thinking
  register cards (Your thinking / Your solution).
- view.ts: the problem pose joins the placeholder fork (the ack never
  flips the part); practice-thinking markers project as data parts;
  problem-dismissed markers project nothing.
- chat.tsx: activeProblemOf drives the dock branch (question > problem
  > composer), the esc layering gains the problem rung (below question,
  above turn) with its discard confirm + the silent marker route; the
  thinking submissions POST through the session's optimistic appends.

components/session.ts

@@ -98,6 +98,10 @@ export function useChatSession({ chatId, initialMessages, initiallyInFlight }: U
 		(toolCallId: string, answer: QuestionAnswerData) => void model.dismissQuestion(toolCallId, answer),
 		[model],
 	);
+	const dismissProblem = useCallback(
+		(problemCallId: string) => void model.dismissProblem(problemCallId),
+		[model],
+	);
 	const setMessages = useCallback((record: LhMessage[]) => model.setMessages(record), [model]);
 	const adoptRecord = useCallback((record: LhMessage[]) => model.adoptRecord(record), [model]);
 	const stop = useCallback(() => model.stop(), [model]);
@@ -114,6 +118,7 @@ export function useChatSession({ chatId, initialMessages, initiallyInFlight }: U
 		steer,
 		submitAnswer,
 		dismissQuestion,
+		dismissProblem,
 		setMessages,
 		adoptRecord,
 		stop,