feat(stammbaum): add recentreAbove pan helper for the mobile anchor (#703)

recentreAbove recentres on a node and lifts it above the viewBox centre
by a fraction of the zoomed viewBox height, measured against the
auto-zoomed height. On a phone this lands the tapped anchor in the band
above the bottom sheet instead of behind it (AC8). A zero bias is exactly
a legible recentre.

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

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

@@ -8,6 +8,7 @@ import {
 	pinchZoom,
 	stepInertia,
 	recentreOn,
+	recentreAbove,
 	clampPan,
 	cornerView,
 	lerpView,
@@ -188,6 +189,34 @@ describe('recentreOn', () => {
 	});
 });
 
+describe('recentreAbove', () => {
+	const node = { x: 300, y: 200 };
+	const base = { x: 100, y: 100 };
+	const baseH = 800;
+
+	it('matches the auto-zooming recentre when the bias is zero', () => {
+		expect(recentreAbove(node, base, { x: 0, y: 0, z: 2 }, baseH, 0)).toEqual(
+			recentreOn(node, base, { x: 0, y: 0, z: 2 }, true)
+		);
+	});
+
+	it('lifts the node up by a fraction of the zoomed viewBox height (clears the bottom sheet)', () => {
+		// recentreOn centres the node (pan.y = 100) at z=2; the bias adds
+		// 0.3 * (baseH / z) = 0.3 * 400 = 120, so the node sits ~20% from the top.
+		const next = recentreAbove(node, base, { x: 0, y: 0, z: 2 }, baseH, 0.3);
+		expect(next.x).toBe(200);
+		expect(next.z).toBe(2);
+		expect(next.y).toBeCloseTo(100 + 0.3 * (baseH / 2), 6);
+	});
+
+	it('measures the bias against the auto-zoomed height when starting zoomed out', () => {
+		// z=0.4 auto-zooms up to LEGIBLE_ZOOM (1); the bias then uses baseH / 1.
+		const next = recentreAbove(node, base, { x: 0, y: 0, z: 0.4 }, baseH, 0.3);
+		expect(next.z).toBe(LEGIBLE_ZOOM);
+		expect(next.y).toBeCloseTo(100 + 0.3 * baseH, 6);
+	});
+});
+
 describe('clampPan', () => {
 	// Base frame is 1000 x 800.
 	it('forbids panning when the whole tree fits (z <= 1)', () => {

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

@@ -236,3 +236,21 @@ export function recentreOn(
 		z: autoZoom ? clampZoom(Math.max(state.z, LEGIBLE_ZOOM)) : state.z
 	};
 }
+
+/**
+ * Recentre on a node but lift it above the viewBox centre by `biasFraction` of
+ * the zoomed viewBox height, so on a phone the tapped anchor lands in the band
+ * above the bottom sheet rather than behind it (#703 AC8). The bias is measured
+ * against the auto-zoomed height ({@link recentreOn} may snap zoom up to
+ * {@link LEGIBLE_ZOOM}), and a zero bias is exactly a legible recentre.
+ */
+export function recentreAbove(
+	nodeCentre: { x: number; y: number },
+	baseCentre: { x: number; y: number },
+	state: PanZoomState,
+	baseH: number,
+	biasFraction: number
+): PanZoomState {
+	const centred = recentreOn(nodeCentre, baseCentre, state, true);
+	return { ...centred, y: centred.y + biasFraction * (baseH / centred.z) };
+}