fix(stammbaum): order keyboard tab stops by visual layout, not DB order (#718)

Person nodes rendered in `nodes` array order (backend/DB row order), so
Tab focus hopped between nodes unrelated to their on-screen position,
failing WCAG 2.4.3 Focus Order (Level A).

Render the node loop in reading order instead: sort by layout y (top
generation first) then x (left-to-right within a row), via a
`nodesInReadingOrder` derived. Nodes without a layout position sort last
(mirroring the `{#if pos}` guard); node.id is the final tie-break for a
total, deterministic comparator. Shift+Tab and reload-stability fall out
for free (reversed render order; x/y independent of backend order).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

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

@@ -70,6 +70,31 @@ let {
 
 const layout = $derived.by<Layout>(() => buildLayout(nodes, edges));
 
+// Keyboard tab order follows DOM source order (#718). Render the nodes in
+// reading order — top generation first (by layout y), left-to-right within a
+// row (by layout x) — so focus sweeps the visual grid instead of the backend
+// row order in which `nodes` arrives. Nodes without a layout position sort last
+// (mirroring the `{#if pos}` render guard); node.id is the final tie-break for a
+// total, deterministic comparator.
+//
+// LOAD-BEARING: relies on every node in a generation sharing the same y
+// (buildLayout assigns y = g * (NODE_H + ROW_GAP)). If the layout is ever
+// changed to stagger nodes vertically within a generation, this y-then-x sort
+// would silently regress tab order — revisit here.
+const nodesInReadingOrder = $derived.by(() =>
+	[...nodes].sort((a, b) => {
+		const pa = layout.positions.get(a.id);
+		const pb = layout.positions.get(b.id);
+		const ay = pa ? pa.y : Infinity;
+		const by = pb ? pb.y : Infinity;
+		if (ay !== by) return ay - by;
+		const ax = pa ? pa.x : Infinity;
+		const bx = pb ? pb.x : Infinity;
+		if (ax !== bx) return ax - bx;
+		return a.id.localeCompare(b.id);
+	})
+);
+
 // Lineage highlight (#703). The adjacency index is rebuilt only when the edges
 // change; the cheap walk re-runs whenever the selection changes. A null
 // highlight (no selection) means full strength everywhere — nothing dims.
@@ -312,8 +337,8 @@ function handleCanvasKey(event: KeyboardEvent) {
 			isConnectorActive={isConnectorActive}
 		/>
 
-		<!-- Nodes -->
-		{#each nodes as node (node.id)}
+		<!-- Nodes (reading order so keyboard Tab follows the visual grid, #718) -->
+		{#each nodesInReadingOrder as node (node.id)}
 			{@const pos = layout.positions.get(node.id)}
 			{#if pos}
 				<StammbaumNode