feat(stammbaum): add clampZoom with resolved 0.25–3.0 zoom bounds (#692)

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/src/lib/person/genealogy/panZoom.ts

@@ -0,0 +1,19 @@
+/**
+ * Pan/zoom geometry for the Stammbaum canvas (#692).
+ *
+ * The Stammbaum renders zoom by deriving the SVG `viewBox` rather than applying
+ * a CSS transform (see `StammbaumTree.svelte`). This module is the single source
+ * of truth for the zoom bounds, the view-state shape, and every pure geometry
+ * helper used by the gesture action, the URL serialiser, and the page. Keeping
+ * the math here (and free of DOM access) makes it unit-testable in the node
+ * project. See ADR-026 for why this is custom rather than a third-party library.
+ */
+
+/** Resolved zoom bounds (OQ-001). */
+export const MIN_ZOOM = 0.25;
+export const MAX_ZOOM = 3.0;
+
+/** Clamp a zoom factor into the supported range. */
+export function clampZoom(z: number): number {
+	return Math.min(MAX_ZOOM, Math.max(MIN_ZOOM, z));
+}