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.

tests/question-core.test.ts

@@ -19,6 +19,9 @@ import {
 	quizInputProblems,
 	quizInputSchema,
 	resolveCorrect,
+	toggleOption,
+	toggleOther,
+	emptyDockSelection,
 	shuffleOptions,
 	type QuestionData,
 	toClientQuestion,
@@ -434,3 +437,50 @@ describe("makeId", () => {
 		assert.equal(ids.size, 200);
 	});
 });
+
+// ─── the dock selection state machine (multi-select Other coexistence) ──
+// The live bug: in a multi-select ask, picking the custom "Other…" row
+// cleared every already-picked option (single-select displacement logic
+// applied to both modes). The machine lives here, pure and pinned.
+describe("dock selection (toggleOption / toggleOther)", () => {
+	it("multi-select: options and Other coexist — neither clears the other", () => {
+		let sel = emptyDockSelection();
+		sel = toggleOption("multi-select", sel, 0);
+		sel = toggleOption("multi-select", sel, 1);
+		assert.deepEqual([...sel.selected], [0, 1]);
+		// Picking Other keeps the sibling selections.
+		sel = toggleOther("multi-select", sel);
+		assert.deepEqual([...sel.selected], [0, 1], "Other never clears picked options");
+		assert.equal(sel.otherSelected, true);
+		// Picking another option keeps Other selected.
+		sel = toggleOption("multi-select", sel, 2);
+		assert.equal(sel.otherSelected, true, "options never clear a typed custom answer");
+		assert.deepEqual([...sel.selected], [0, 1, 2]);
+		// Deselecting an option toggles it alone.
+		sel = toggleOption("multi-select", sel, 0);
+		assert.deepEqual([...sel.selected], [1, 2]);
+		// Other toggles off alone; the siblings survive.
+		sel = toggleOther("multi-select", sel);
+		assert.equal(sel.otherSelected, false);
+		assert.deepEqual([...sel.selected], [1, 2]);
+	});
+
+	it("multi-select: any selection clears dont-know", () => {
+		let sel = { selected: new Set<number>(), otherSelected: false, dontKnow: true };
+		sel = toggleOption("multi-select", sel, 0);
+		assert.equal(sel.dontKnow, false);
+		sel = toggleOther("multi-select", sel);
+		assert.equal(sel.dontKnow, false);
+	});
+
+	it("single-select: displacement semantics are unchanged", () => {
+		let sel = emptyDockSelection();
+		sel = toggleOption("single-select", sel, 0);
+		sel = toggleOther("single-select", sel);
+		assert.deepEqual([...sel.selected], [], "Other displaces the picked option");
+		assert.equal(sel.otherSelected, true);
+		sel = toggleOption("single-select", sel, 1);
+		assert.equal(sel.otherSelected, false, "an option displaces Other");
+		assert.deepEqual([...sel.selected], [1]);
+	});
+});