feat(stammbaum): add familyForest.pickStructuralOwner (#724)

Structural-owner rule for couples: earlier birth year wins, missing year sorts
last, ties break on stable id. The single definition reused by the cross-link,
cycle and intra-family paths.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/src/lib/person/genealogy/layout/familyForest.test.ts

@@ -0,0 +1,23 @@
+import { describe, it, expect } from 'vitest';
+import { pickStructuralOwner } from './familyForest';
+
+type Person = { id: string; birthYear?: number };
+
+describe('pickStructuralOwner', () => {
+	const p = (id: string, birthYear?: number): Person => ({ id, birthYear });
+
+	it('picks the earlier-born spouse as structural owner', () => {
+		expect(pickStructuralOwner(p('a', 1900), p('b', 1920))).toBe('a');
+		expect(pickStructuralOwner(p('a', 1920), p('b', 1900))).toBe('b');
+	});
+
+	it('sorts a missing birthYear last (the dated spouse owns)', () => {
+		expect(pickStructuralOwner(p('a'), p('b', 1900))).toBe('b');
+		expect(pickStructuralOwner(p('a', 1900), p('b'))).toBe('a');
+	});
+
+	it('breaks ties on stable id when birth years match or are both missing', () => {
+		expect(pickStructuralOwner(p('zzz', 1900), p('aaa', 1900))).toBe('aaa');
+		expect(pickStructuralOwner(p('zzz'), p('aaa'))).toBe('aaa');
+	});
+});

frontend/src/lib/person/genealogy/layout/familyForest.ts

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+// Domain-aware construction of the genealogy "family forest" (#724).
+//
+// This module owns every piece of genealogy knowledge the layout needs —
+// spouse runs, birth-year ordering, structural-owner selection, intra-family
+// marriage resolution and cross-links — and flattens it into the abstract
+// { id, width, children } nodes that the domain-agnostic tidyTree.ts packs.
+// buildLayout.ts orchestrates the two and maps run positions back to persons.
+
+import type { components } from '$lib/generated/api';
+
+type PersonNodeDTO = components['schemas']['PersonNodeDTO'];
+
+/**
+ * Choose the structural owner of a couple: the spouse who keeps the bloodline
+ * (hierarchy) position. Earlier birth year wins; a missing birth year sorts
+ * last; ties break on the stable id. Shared by the cycle, cross-link and
+ * intra-family paths so the rule is defined exactly once.
+ */
+export function pickStructuralOwner(
+	a: { id: string; birthYear?: number | null },
+	b: { id: string; birthYear?: number | null }
+): string {
+	const ra = ownerRank(a);
+	const rb = ownerRank(b);
+	if (ra !== rb) return ra < rb ? a.id : b.id;
+	return a.id <= b.id ? a.id : b.id;
+}
+
+// Lower is "more owning". Missing birth year → +Infinity (sorts last).
+function ownerRank(p: { birthYear?: number | null }): number {
+	return p.birthYear == null ? Number.POSITIVE_INFINITY : p.birthYear;
+}
+
+export type { PersonNodeDTO };