fix: multi-select Other coexistence + the malformed-assessment probe-done crash

Two live bugs from the modal-realism stress runs:

1. Multi-select ask: picking the custom 'Other…' row cleared every
   already-picked option, and picking any option cleared a typed custom
   answer — single-select displacement logic applied to both modes. The
   dock's selection is now a pure state machine in question-core
   (toggleOption/toggleOther over DockSelection, unit-tested): single
   displaces (unchanged), multi coexists — neither channel clears the
   other; the typed text always survives.

2. ProbeDoneCard crashed on a REJECTED probe_done call: the model
   stringified 'assessment' (the same double-encoding habit as the
   options bug), zod rejected the call, the model retried — but the
   card's input-preview fallback trusted the errored part's unvalidated
   input shape and died on assessment.strands.map. The preview now reads
   input only while the call is pending or streaming (guarded with
   Array.isArray); an output-error part renders a quiet
   malformed-and-retried note instead of the garbage.

Pinned: question-core.test.ts (3 dock-selection machine cases),
render-cards.test.tsx (4 probe-done card cases incl. the exact live
stringified shape, plus the multi-select ask markup pin).

Gates: tsc, lint, build, 315/315.

src/core/question-core.ts

@@ -278,6 +278,44 @@ export interface AnswerRequest {
 	note?: string;
 }
 
+// ─── dock selection (the client state machine, pure for tests) ─────────────
+
+/** The question dock's selection state (0-based indices in display order). */
+export interface DockSelection {
+	selected: Set<number>;
+	otherSelected: boolean;
+	dontKnow: boolean;
+}
+
+export const emptyDockSelection = (): DockSelection => ({ selected: new Set(), otherSelected: false, dontKnow: false });
+
+/**
+ * Toggling a listed option. Single-select: the option becomes THE answer and
+ * displaces "Other". Multi-select: the option toggles alongside its siblings
+ * AND "Other" — selections are independent (picking options must never clear
+ * a custom answer, and picking the custom row must never clear them).
+ */
+export function toggleOption(mode: SelectMode, sel: DockSelection, idx: number): DockSelection {
+	if (mode === "multi-select") {
+		const selected = new Set(sel.selected);
+		if (selected.has(idx)) selected.delete(idx);
+		else selected.add(idx);
+		return { selected, otherSelected: sel.otherSelected, dontKnow: false };
+	}
+	return { selected: new Set([idx]), otherSelected: false, dontKnow: false };
+}
+
+/**
+ * Toggling the ask "Other…" row. Single-select: picking it makes it THE
+ * answer, displacing the listed options (the typed text always survives).
+ * Multi-select: it toggles alongside the listed selections and clears
+ * nothing.
+ */
+export function toggleOther(mode: SelectMode, sel: DockSelection): DockSelection {
+	if (mode === "multi-select") return { ...sel, otherSelected: !sel.otherSelected, dontKnow: false };
+	return { selected: new Set(), otherSelected: true, dontKnow: false };
+}
+
 export const QUESTION_ENTRY = "lh_question";
 export const QUESTION_ANSWERED_ENTRY = "lh_question_answered";