feat(stammbaum): recentre-on-node with legible auto-zoom (#692)

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

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

@@ -14,6 +14,9 @@ export const MIN_ZOOM = 0.25;
 export const MAX_ZOOM = 3.0;
 export const DEFAULT_ZOOM = 1;
 
+/** Minimum zoom a recentre will snap up to so the focal node's text is legible (OQ-005). */
+export const LEGIBLE_ZOOM = 1;
+
 /**
  * The canvas view state. `x`/`y` are pan offsets applied to the viewBox centre
  * (SVG user units); `z` is the zoom factor. The default `{0, 0, 1}` frames the
@@ -106,3 +109,23 @@ export function zoomAtPoint(
 		z
 	};
 }
+
+/**
+ * Pan so a node sits at the viewBox centre (US-PAN-005). Because the viewBox
+ * centre is `baseCentre + pan` independent of zoom, centring is a pure pan:
+ * `pan = nodeCentre - baseCentre`. When `autoZoom` is set, a zoomed-out view is
+ * snapped up to {@link LEGIBLE_ZOOM} so the focal node's text is readable
+ * (OQ-005); an already-legible zoom is preserved.
+ */
+export function recentreOn(
+	nodeCentre: { x: number; y: number },
+	baseCentre: { x: number; y: number },
+	state: PanZoomState,
+	autoZoom: boolean
+): PanZoomState {
+	return {
+		x: nodeCentre.x - baseCentre.x,
+		y: nodeCentre.y - baseCentre.y,
+		z: autoZoom ? clampZoom(Math.max(state.z, LEGIBLE_ZOOM)) : state.z
+	};
+}