test+fix(stammbaum): capture script floors >= 1 multi-spouse person (#361)

@Markus + @Tobias + @Sara on PR #693: the multi-spouse property is
load-bearing for buildLayout.test.ts (canonical_fixture_assigns_a_position
_to_every_node_with_multiple_spouses + canonical_fixture_multi_spouse
_falls_through_to_displayName_when_no_fromYear). A recapture against a
dataset that lost every multi-spouse person would silently degrade those
tests to vacuous truth.

Add MIN_MULTI_SPOUSE_PERSONS=1 to the capture-script sanity gates. Extract
the validator into a unit-testable TS module next to the fixture; the .mjs
script keeps its inline copy (one-file local utility) but the contract is
now covered by validateFixture.test.ts.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

frontend/src/lib/person/genealogy/__fixtures__/validateFixture.test.ts

@@ -0,0 +1,62 @@
+import { describe, it, expect } from 'vitest';
+import { validateFixture } from './validateFixture';
+import canonicalFixture from './stammbaum.json';
+
+// The fixture validator is the load-bearing contract for the canonical
+// Stammbaum snapshot: every gate here corresponds to an invariant that
+// buildLayout.test.ts relies on. Adding or removing a gate without updating
+// these tests is the failure we want to catch.
+
+function networkWithNodes(count: number) {
+	return {
+		nodes: Array.from({ length: count }, (_, i) => ({ id: `n${i}`, generation: i % 6 })),
+		edges: []
+	};
+}
+
+function spouseEdge(a: string, b: string) {
+	return {
+		id: `${a}|${b}`,
+		personId: a,
+		relatedPersonId: b,
+		personDisplayName: '',
+		relatedPersonDisplayName: '',
+		relationType: 'SPOUSE_OF'
+	};
+}
+
+describe('validateFixture', () => {
+	it('passes_for_the_canonical_fixture', () => {
+		expect(() => validateFixture(canonicalFixture)).not.toThrow();
+	});
+
+	it('rejects_a_fixture_below_the_min_node_floor', () => {
+		expect(() => validateFixture(networkWithNodes(10))).toThrow(/>= 50 nodes/);
+	});
+
+	it('rejects_a_fixture_with_no_multi_spouse_person', () => {
+		// 50 nodes, 5 generations, several SPOUSE_OF edges — but every spouse
+		// edge connects a different pair, so nobody has more than one partner.
+		// Without the multi-spouse floor this would silently pass and the
+		// canonical_fixture_assigns_a_position_to_every_node_with_multiple_spouses
+		// test in buildLayout.test.ts would degrade to vacuous truth.
+		const nodes = Array.from({ length: 50 }, (_, i) => ({ id: `n${i}`, generation: i % 5 }));
+		const edges = [spouseEdge('n0', 'n1'), spouseEdge('n2', 'n3'), spouseEdge('n4', 'n5')];
+		expect(() => validateFixture({ nodes, edges })).toThrow(
+			/>= 1 person with multiple SPOUSE_OF edges/
+		);
+	});
+
+	it('accepts_a_fixture_where_one_person_has_multiple_spouse_edges', () => {
+		const nodes = Array.from({ length: 50 }, (_, i) => ({ id: `n${i}`, generation: i % 5 }));
+		const edges = [spouseEdge('n0', 'n1'), spouseEdge('n0', 'n2')];
+		expect(() => validateFixture({ nodes, edges })).not.toThrow();
+	});
+
+	it('counts_multi_spouse_persons_via_either_edge_direction', () => {
+		const nodes = Array.from({ length: 50 }, (_, i) => ({ id: `n${i}`, generation: i % 5 }));
+		// n5 is the related party in both edges — still counts as multi-spouse.
+		const edges = [spouseEdge('n1', 'n5'), spouseEdge('n2', 'n5')];
+		expect(() => validateFixture({ nodes, edges })).not.toThrow();
+	});
+});