feat(stammbaum): render generation gutter on the family tree (#689)

The gutter sits 100 px to the left of the tree canvas on md+ viewports
(hidden entirely below md to preserve scrollable area on phones — see
spec's deliberate dual-audience trade-off). Per occupied generation
row it draws:

- A full-width decorative stripe rect alternating transparent and
  var(--c-gutter-stripe). aria-hidden because it carries no meaning.
- The label `G{n}` at the left edge, sourced from the un-shifted
  node.generation value (never the post-normalise rank), wrapped in
  `<g role="text" aria-label="Generation N">` so screen readers
  announce the full word instead of "G three".

CSS adds --c-gutter-stripe in both the light root and the dark mode
blocks (8% / 14% mint over canvas — decorative contrast carve-out).

Browser tests cover label rendering, the ARIA wrapper, and the
viewport-below-md absent-gutter path via a matchMedia stub. Existing
StammbaumTree structural-invariant tests still pass since none of
them assert anything inside the gutter region.

Refs #689

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

frontend/src/lib/person/genealogy/StammbaumTree.svelte

@@ -1,7 +1,13 @@
 <script lang="ts">
 import { SvelteMap, SvelteSet } from 'svelte/reactivity';
 import type { components } from '$lib/generated/api';
-import { buildLayout, NODE_W, NODE_H, type Layout } from '$lib/person/genealogy/layout/buildLayout';
+import {
+	buildLayout,
+	NODE_W,
+	NODE_H,
+	ROW_GAP,
+	type Layout
+} from '$lib/person/genealogy/layout/buildLayout';
 
 type PersonNodeDTO = components['schemas']['PersonNodeDTO'];
 type RelationshipDTO = components['schemas']['RelationshipDTO'];
@@ -17,10 +23,52 @@ interface Props {
 let { nodes, edges, selectedId, zoom, onSelect }: Props = $props();
 
 const layout = $derived.by<Layout>(() => buildLayout(nodes, edges));
+
+// Stammbaum gutter (#689). 100 px column on the left of the canvas on md+
+// viewports, carrying the G{n} label per generation row. Hidden entirely on
+// phones (canvas is already overflow-scroll; 100 px of permanent chrome is
+// too costly on a 320 px screen).
+const GUTTER_WIDTH_DESKTOP = 100;
+const GUTTER_MEDIA_QUERY = '(min-width: 768px)';
+let isMdOrUp = $state(false);
+$effect(() => {
+	if (typeof window === 'undefined' || typeof window.matchMedia !== 'function') return;
+	const mq = window.matchMedia(GUTTER_MEDIA_QUERY);
+	isMdOrUp = mq.matches;
+	const handler = (e: MediaQueryListEvent) => (isMdOrUp = e.matches);
+	mq.addEventListener('change', handler);
+	return () => mq.removeEventListener('change', handler);
+});
+const gutterWidth = $derived(isMdOrUp ? GUTTER_WIDTH_DESKTOP : 0);
+
+type GutterRow = { rank: number; y: number; label: number | null };
+const gutterRows = $derived.by<GutterRow[]>(() => {
+	if (gutterWidth === 0) return [];
+	const byId = new SvelteMap(nodes.map((n) => [n.id, n]));
+	const rows: GutterRow[] = [];
+	const sortedRanks = [...layout.generations.keys()].sort((a, b) => a - b);
+	for (const rank of sortedRanks) {
+		const ids = layout.generations.get(rank)!;
+		const firstPos = layout.positions.get(ids[0]);
+		if (!firstPos) continue;
+		let label: number | null = null;
+		for (const id of ids) {
+			const g = byId.get(id)?.generation;
+			if (g != null) {
+				label = g;
+				break;
+			}
+		}
+		rows.push({ rank, y: firstPos.y, label });
+	}
+	return rows;
+});
+
 const viewBox = $derived.by(() => {
-	const w = layout.viewW / zoom;
+	const totalW = layout.viewW + gutterWidth;
+	const w = totalW / zoom;
 	const h = layout.viewH / zoom;
-	const cx = layout.viewX + layout.viewW / 2;
+	const cx = layout.viewX - gutterWidth + totalW / 2;
 	const cy = layout.viewY + layout.viewH / 2;
 	return `${cx - w / 2} ${cy - h / 2} ${w} ${h}`;
 });
@@ -117,6 +165,44 @@ const parentLinks = $derived.by<ParentLinks>(() => {
 	aria-label="Stammbaum"
 	class="block h-full w-full"
 >
+	<!-- Gutter stripe underlay (#689) — decorative full-row bands alternating
+	     transparent / var(--c-gutter-stripe). aria-hidden because they carry
+	     no meaning; the row's generation is announced by the label group below. -->
+	{#each gutterRows as row, i (`stripe-${row.rank}`)}
+		<rect
+			aria-hidden="true"
+			x={layout.viewX - gutterWidth}
+			y={row.y - ROW_GAP / 2}
+			width={layout.viewW + gutterWidth}
+			height={NODE_H + ROW_GAP}
+			fill={i % 2 === 0 ? 'transparent' : 'var(--c-gutter-stripe)'}
+		/>
+	{/each}
+
+	<!-- Gutter labels (#689) — `G{node.generation}` per occupied row at the
+	     un-shifted source-truth value. Wrapped in <g role="text"> so screen
+	     readers announce "Generation three" instead of "G three". -->
+	{#each gutterRows as row (`label-${row.rank}`)}
+		{#if row.label != null}
+			<g role="text" aria-label={`Generation ${row.label}`}>
+				<text
+					x={layout.viewX - gutterWidth + 12}
+					y={row.y + NODE_H / 2}
+					text-anchor="start"
+					dominant-baseline="middle"
+					font-family="var(--font-sans)"
+					font-size="12"
+					font-weight="700"
+					letter-spacing="0.08em"
+					fill="var(--c-ink-2)"
+					style:text-transform="uppercase"
+				>
+					G{row.label}
+				</text>
+			</g>
+		{/if}
+	{/each}
+
 	<!-- Shared parent-pair → children: drop from spouse midpoint to a sibling
 	     bar, then short verticals from the bar to each child top. -->
 	{#each parentLinks.shared as group (group.key)}

frontend/src/lib/person/genealogy/StammbaumTree.svelte.test.ts

@@ -648,3 +648,69 @@ describe('StammbaumTree node rendering branches', () => {
 		expect(accentRects.length).toBe(1);
 	});
 });
+
+describe('StammbaumTree generation gutter (#689)', () => {
+	it('renders a G{n} label per occupied generation row when at least one node carries generation', async () => {
+		render(StammbaumTree, {
+			nodes: [
+				{ id: ID_A, displayName: 'Walter', familyMember: true, generation: 2 },
+				{ id: ID_B, displayName: 'Herbert', familyMember: true, generation: 3 }
+			],
+			edges: [],
+			selectedId: null,
+			zoom: 1,
+			onSelect: () => {}
+		});
+
+		const labels = Array.from(document.querySelectorAll('g[role="text"]')).map((g) =>
+			g.getAttribute('aria-label')
+		);
+		expect(labels).toContain('Generation 2');
+		expect(labels).toContain('Generation 3');
+	});
+
+	it('wraps the visible G3 text inside an aria-labelled group so screen readers announce "Generation"', async () => {
+		render(StammbaumTree, {
+			nodes: [{ id: ID_A, displayName: 'Herbert', familyMember: true, generation: 3 }],
+			edges: [],
+			selectedId: null,
+			zoom: 1,
+			onSelect: () => {}
+		});
+
+		const g3 = Array.from(document.querySelectorAll('g[role="text"]')).find(
+			(g) => g.getAttribute('aria-label') === 'Generation 3'
+		);
+		expect(g3).toBeDefined();
+		expect(g3!.querySelector('text')!.textContent).toMatch(/G\s*3/);
+	});
+
+	it('omits the gutter when matchMedia (min-width: 768px) is false', async () => {
+		const originalMatchMedia = window.matchMedia;
+		window.matchMedia = ((query: string) => ({
+			matches: false,
+			media: query,
+			onchange: null,
+			addEventListener: () => {},
+			removeEventListener: () => {},
+			addListener: () => {},
+			removeListener: () => {},
+			dispatchEvent: () => false
+		})) as unknown as typeof window.matchMedia;
+
+		try {
+			render(StammbaumTree, {
+				nodes: [{ id: ID_A, displayName: 'Herbert', familyMember: true, generation: 3 }],
+				edges: [],
+				selectedId: null,
+				zoom: 1,
+				onSelect: () => {}
+			});
+
+			const labelGroups = Array.from(document.querySelectorAll('g[role="text"]'));
+			expect(labelGroups).toHaveLength(0);
+		} finally {
+			window.matchMedia = originalMatchMedia;
+		}
+	});
+});

frontend/src/routes/layout.css

@@ -160,6 +160,12 @@
 	   with axe (tracked in #480) before tweaking the palette. */
 	--timeline-bar-idle: rgba(161, 220, 216, 0.35);
 	--timeline-bar-outside: var(--c-line);
+
+	/* Stammbaum gutter stripe (issue #689) — decorative full-row underlay
+	   alternating with transparent. Mint-tinted on canvas to mark generation
+	   rows without competing with node fills. 8% on light surface ≈ #ECF6F4
+	   (~1.04:1 vs canvas — decorative carve-out). */
+	--c-gutter-stripe: rgba(161, 220, 216, 0.08);
 }
 
 /* ─── 5. Dark mode ─────────────────────────────────────────────────────────── */
@@ -236,6 +242,10 @@
 		   clears WCAG 1.4.11 non-text contrast for large UI elements. */
 		--timeline-bar-idle: #3a6e8c;
 		--timeline-bar-outside: #1a2735;
+
+		/* Stammbaum gutter stripe (issue #689) — 14% mint on dark canvas for
+		   visibility parity with the 8% light-mode token. Decorative carve-out. */
+		--c-gutter-stripe: rgba(161, 220, 216, 0.14);
 	}
 }
 
@@ -308,6 +318,9 @@
 	   clears WCAG 1.4.11 non-text contrast for large UI elements. */
 	--timeline-bar-idle: #3a6e8c;
 	--timeline-bar-outside: #1a2735;
+
+	/* Stammbaum gutter stripe (issue #689) — KEEP IN SYNC with the @media block. */
+	--c-gutter-stripe: rgba(161, 220, 216, 0.14);
 }
 
 /* ─── 6. Icon inversion — De Gruyter icons are black SVGs loaded as <img> ──── */