feat(stammbaum): seed layout rank from imported generation (#689)

buildLayout switches to a two-stage assignment:

1. Seed — every node with node.generation != null is locked at that
   rank. The fallback heuristic never moves a locked rank, and the
   spouse-pulldown never pulls a locked rank.
2. Fallback — for unseeded nodes, rank = max(parent rank) + 1 reading
   parents from the same unified rank map, so an unseeded child of a
   seeded G 2 parent correctly inherits rank 3. Spouse-pulldown ties
   unseeded spouses to their deeper partner exactly as before.
3. Normalise — if any rank is negative (future G −1 ancestor), shift
   the whole map so min(rank) == 0. No-op for today's data.

Fixes the Herbert Cram pattern from #361's review: two parented
spouses with imported G 3 now render on the same y row. Existing
StammbaumTree tests still pass byte-for-byte because every test node
has node.generation undefined, so the heuristic runs unchanged.

Refs #689

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

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

@@ -0,0 +1,113 @@
+import { describe, it, expect } from 'vitest';
+import { buildLayout, NODE_H, ROW_GAP } from './buildLayout';
+import type { components } from '$lib/generated/api';
+
+type PersonNodeDTO = components['schemas']['PersonNodeDTO'];
+type RelationshipDTO = components['schemas']['RelationshipDTO'];
+
+const PARENT = '00000000-0000-0000-0000-000000000001';
+const CHILD = '00000000-0000-0000-0000-000000000002';
+const SPOUSE_A = '00000000-0000-0000-0000-000000000003';
+const SPOUSE_B = '00000000-0000-0000-0000-000000000004';
+const NEGATIVE_A = '00000000-0000-0000-0000-000000000005';
+const NEGATIVE_B = '00000000-0000-0000-0000-000000000006';
+const NEGATIVE_C = '00000000-0000-0000-0000-000000000007';
+
+function node(id: string, displayName: string, generation: number | null = null): PersonNodeDTO {
+	return generation == null
+		? { id, displayName, familyMember: true }
+		: { id, displayName, familyMember: true, generation };
+}
+
+function parentEdge(parentId: string, childId: string, id = parentId + childId): RelationshipDTO {
+	return {
+		id,
+		personId: parentId,
+		relatedPersonId: childId,
+		personDisplayName: '',
+		relatedPersonDisplayName: '',
+		relationType: 'PARENT_OF'
+	};
+}
+
+function spouseEdge(a: string, b: string, id = a + b): RelationshipDTO {
+	return {
+		id,
+		personId: a,
+		relatedPersonId: b,
+		personDisplayName: '',
+		relatedPersonDisplayName: '',
+		relationType: 'SPOUSE_OF'
+	};
+}
+
+function yOf(layout: ReturnType<typeof buildLayout>, id: string): number {
+	const p = layout.positions.get(id);
+	if (!p) throw new Error(`No position for ${id}`);
+	return p.y;
+}
+
+describe('buildLayout — generation seeding (#689)', () => {
+	it('Herbert Cram regression: two parented G=3 spouses share the same row', () => {
+		// Both Herbert (G 3) and Clara (G 3) are parented children of their respective
+		// G 2 ancestors. They are spouses. Before #689 the iterative longest-path put
+		// Herbert one row deeper than Clara via the spouse-pulldown of his loose parent.
+		// With imported generation as a strict seed both render at the same y.
+		const layout = buildLayout(
+			[node(SPOUSE_A, 'Herbert', 3), node(SPOUSE_B, 'Clara', 3)],
+			[spouseEdge(SPOUSE_A, SPOUSE_B)]
+		);
+
+		expect(yOf(layout, SPOUSE_A)).toBe(yOf(layout, SPOUSE_B));
+	});
+
+	it('strict-seed override: imported generation pins rank even when parent edges imply deeper', () => {
+		// PARENT has no explicit generation → falls back to 0. CHILD is parented under
+		// PARENT but has imported generation = 3. The seeded rank wins; the heuristic
+		// must not push CHILD to rank 1.
+		const layout = buildLayout(
+			[node(PARENT, 'Parent'), node(CHILD, 'Child', 3)],
+			[parentEdge(PARENT, CHILD)]
+		);
+
+		expect(yOf(layout, CHILD)).toBe(3 * (NODE_H + ROW_GAP));
+	});
+
+	it('fallback inherits seeded parent rank: G 2 parent → null-gen child lands at rank 3', () => {
+		// CHILD has no imported generation. PARENT has generation = 2. The fallback
+		// reads PARENT's rank from the unified rank map (2) and computes 2 + 1 = 3.
+		const layout = buildLayout(
+			[node(PARENT, 'Parent', 2), node(CHILD, 'Child')],
+			[parentEdge(PARENT, CHILD)]
+		);
+
+		expect(yOf(layout, CHILD)).toBe(3 * (NODE_H + ROW_GAP));
+	});
+
+	it('normalise is a no-op when all ranks are non-negative', () => {
+		// Seeded ranks [3, 4, 5] → y must reflect [3, 4, 5] without any shift.
+		const G3 = '00000000-0000-0000-0000-000000000031';
+		const G4 = '00000000-0000-0000-0000-000000000032';
+		const G5 = '00000000-0000-0000-0000-000000000033';
+		const layout = buildLayout(
+			[node(G3, 'three', 3), node(G4, 'four', 4), node(G5, 'five', 5)],
+			[]
+		);
+
+		expect(yOf(layout, G3)).toBe(3 * (NODE_H + ROW_GAP));
+		expect(yOf(layout, G4)).toBe(4 * (NODE_H + ROW_GAP));
+		expect(yOf(layout, G5)).toBe(5 * (NODE_H + ROW_GAP));
+	});
+
+	it('normalise shifts negative seeds so min rank becomes 0', () => {
+		// Seeded ranks [-1, 0, 1] → after shift they render at [0, 1, 2] y-rows.
+		const layout = buildLayout(
+			[node(NEGATIVE_A, 'minus-one', -1), node(NEGATIVE_B, 'zero', 0), node(NEGATIVE_C, 'one', 1)],
+			[]
+		);
+
+		expect(yOf(layout, NEGATIVE_A)).toBe(0);
+		expect(yOf(layout, NEGATIVE_B)).toBe(1 * (NODE_H + ROW_GAP));
+		expect(yOf(layout, NEGATIVE_C)).toBe(2 * (NODE_H + ROW_GAP));
+	});
+});

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

@@ -38,49 +38,71 @@ export function buildLayout(allNodes: PersonNodeDTO[], allEdges: RelationshipDTO
 		}
 	}
 
-	// Iterative longest-path generation assignment.
+	// Two-stage rank assignment (#689):
 	//
-	// Each node's generation = max(parent generations) + 1 (roots stay at 0).
-	// Then spouses are pulled to share the deeper generation. Pulling a spouse
-	// down can shift their own descendants, so we iterate until stable rather
-	// than running BFS once like the previous implementation (which left
-	// e.g. a child of a "later-pulled" spouse stranded one row too high).
-	const generation = new Map<string, number>();
-	for (const n of allNodes) generation.set(n.id, 0);
+	// 1. Seed: every node with imported generation is locked at that rank.
+	//    The fallback heuristic never moves a locked rank, and spouse-pulldown
+	//    never pulls a locked rank.
+	// 2. Fallback: for the remaining (unseeded) nodes, rank = max(parent rank)
+	//    + 1, reading parent rank from the same unified map so an unseeded
+	//    child of a seeded G 2 parent correctly inherits rank 3. Spouse-
+	//    pulldown ties unseeded spouses to their deeper partner.
+	// 3. Normalise: if any seeded rank is negative (a future G −1 ancestor),
+	//    shift the entire map so min(rank) == 0. No-op fast path covers
+	//    today's data.
+	const rank = new Map<string, number>();
+	const locked = new Set<string>();
+	for (const n of allNodes) {
+		if (n.generation != null) {
+			rank.set(n.id, n.generation);
+			locked.add(n.id);
+		} else {
+			rank.set(n.id, 0);
+		}
+	}
 	const maxIters = allNodes.length + 4;
 	for (let it = 0; it < maxIters; it++) {
 		let changed = false;
 		for (const n of allNodes) {
+			if (locked.has(n.id)) continue;
 			const parents = childToParents.get(n.id) ?? [];
 			if (parents.length === 0) continue;
-			let maxParentGen = -1;
+			let maxParentRank = -Infinity;
 			for (const pid of parents) {
-				maxParentGen = Math.max(maxParentGen, generation.get(pid) ?? 0);
+				maxParentRank = Math.max(maxParentRank, rank.get(pid) ?? 0);
 			}
-			const newGen = maxParentGen + 1;
-			if ((generation.get(n.id) ?? 0) < newGen) {
-				generation.set(n.id, newGen);
+			const newRank = maxParentRank + 1;
+			if ((rank.get(n.id) ?? 0) < newRank) {
+				rank.set(n.id, newRank);
 				changed = true;
 			}
 		}
 		for (const [a, b] of spousePairs) {
-			const m = Math.max(generation.get(a) ?? 0, generation.get(b) ?? 0);
-			if ((generation.get(a) ?? 0) < m) {
-				generation.set(a, m);
+			const ra = rank.get(a) ?? 0;
+			const rb = rank.get(b) ?? 0;
+			const m = Math.max(ra, rb);
+			if (!locked.has(a) && ra < m) {
+				rank.set(a, m);
 				changed = true;
 			}
-			if ((generation.get(b) ?? 0) < m) {
-				generation.set(b, m);
+			if (!locked.has(b) && rb < m) {
+				rank.set(b, m);
 				changed = true;
 			}
 		}
 		if (!changed) break;
 	}
+	let minRank = Infinity;
+	for (const r of rank.values()) minRank = Math.min(minRank, r);
+	if (minRank < 0) {
+		const shift = -minRank;
+		for (const [id, r] of rank) rank.set(id, r + shift);
+	}
 
-	// Group by generation, then sort within generation by display name.
+	// Group by rank, then sort within rank by display name.
 	const generations = new Map<number, string[]>();
 	for (const n of allNodes) {
-		const g = generation.get(n.id) ?? 0;
+		const g = rank.get(n.id) ?? 0;
 		if (!generations.has(g)) generations.set(g, []);
 		generations.get(g)!.push(n.id);
 	}