Initial commit: webtrees full diagram chart module

Interactive SVG family tree visualization using ELK (Sugiyama) for
layout and D3 for rendering. Shows ancestors, descendants, and siblings
in a single diagram with orthogonal bus-line connectors.

Features:
- Bidirectional tree traversal (ancestors + descendants + siblings)
- Generation-aligned layout with post-processing Y-snap
- Person cards with photos, names, dates, and hover bio cards
- "More ancestors" indicator for persons with hidden parents
- Pan/zoom navigation
- Docker dev environment

src/Model/FamilyNode.php

@@ -0,0 +1,39 @@
+<?php
+
+declare(strict_types=1);
+
+namespace FullDiagram\Model;
+
+use JsonSerializable;
+
+class FamilyNode implements JsonSerializable
+{
+    /**
+     * @param NodeData|null   $spouse
+     * @param list<NodeData>  $children
+     * @param string          $familyXref
+     * @param list<NodeData>  $parents   Used in ancestor context (both parents)
+     */
+    public function __construct(
+        private readonly ?NodeData $spouse,
+        private readonly array     $children = [],
+        private readonly string    $familyXref = '',
+        private readonly array     $parents = [],
+    ) {
+    }
+
+    public function jsonSerialize(): mixed
+    {
+        $data = [
+            'familyXref' => $this->familyXref,
+            'spouse'     => $this->spouse,
+            'children'   => $this->children,
+        ];
+
+        if ($this->parents !== []) {
+            $data['parents'] = $this->parents;
+        }
+
+        return $data;
+    }
+}