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

resources/js/modules/lib/tree/spouse-util.js

@@ -0,0 +1,39 @@
+/**
+ * Shared spouse placement utilities.
+ */
+
+export const SPOUSE_GAP = 10; // px between partner cards
+
+/**
+ * Compute the X offset for spouse placement.
+ *
+ * Alternates right/left with increasing distance:
+ *   family 0 → right  (1 * offset)
+ *   family 1 → left   (-1 * offset)
+ *   family 2 → right  (2 * offset)
+ *   family 3 → left   (-2 * offset)
+ *
+ * @param {number} index - Family index (0-based)
+ * @param {number} cardWidth
+ * @param {number} gap
+ * @returns {number} X offset from person position
+ */
+export function computeSpouseOffset(index, cardWidth, gap) {
+    const unit = cardWidth + gap;
+    const distance = Math.floor(index / 2) + 1;
+    const direction = index % 2 === 0 ? 1 : -1;
+    return direction * distance * unit;
+}
+
+/**
+ * Compute the couple midpoint X offset for a given family index.
+ * This is half the spouse offset (the center between person and spouse).
+ *
+ * @param {number} familyIndex
+ * @param {number} cardWidth
+ * @param {number} gap
+ * @returns {number}
+ */
+export function coupleMidpointOffset(familyIndex, cardWidth, gap) {
+    return computeSpouseOffset(familyIndex, cardWidth, gap) / 2;
+}