refactor(stammbaum): extract + unit-test pinch and inertia math (#692)

Move the pinch-zoom (pinchZoom) and inertia-step (stepInertia) geometry out of
the panZoomGestures DOM glue into pure, unit-tested helpers in panZoom.ts, with
named FRAME_MS/INERTIA_* constants. Addresses the QA blocker that the gesture
module's core math was untested. No behaviour change.

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

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

@@ -5,6 +5,8 @@ import {
 	serializePanZoomParams,
 	screenDeltaToSvg,
 	zoomAtPoint,
+	pinchZoom,
+	stepInertia,
 	recentreOn,
 	clampPan,
 	lerpView,
@@ -133,6 +135,35 @@ describe('zoomAtPoint', () => {
 	});
 });
 
+describe('pinchZoom', () => {
+	it('scales zoom by the finger-distance ratio around the centroid', () => {
+		// Fingers spread 100→200 → 2× zoom; centroid at canvas centre → no pan.
+		expect(pinchZoom({ x: 0, y: 0, z: 1 }, 1, 100, 200, 0, 0)).toEqual({ x: 0, y: 0, z: 2 });
+	});
+
+	it('zooms out when fingers pinch together', () => {
+		expect(pinchZoom({ x: 0, y: 0, z: 2 }, 2, 200, 100, 0, 0).z).toBe(1);
+	});
+
+	it('clamps the scaled zoom into bounds', () => {
+		expect(pinchZoom({ x: 0, y: 0, z: 2 }, 2, 100, 1000, 0, 0).z).toBe(MAX_ZOOM);
+	});
+
+	it('treats a zero start distance as no zoom change', () => {
+		expect(pinchZoom({ x: 5, y: 5, z: 1.5 }, 1.5, 0, 200, 0, 0).z).toBe(1.5);
+	});
+});
+
+describe('stepInertia', () => {
+	it('advances the pan by velocity × frame duration in the drag direction', () => {
+		expect(stepInertia({ x: 100, y: 50, z: 1 }, 0.5, 0.25, 16)).toEqual({ x: 92, y: 46, z: 1 });
+	});
+
+	it('leaves zoom untouched', () => {
+		expect(stepInertia({ x: 0, y: 0, z: 2.5 }, 1, 1, 16).z).toBe(2.5);
+	});
+});
+
 describe('recentreOn', () => {
 	const node = { x: 300, y: 200 };
 	const base = { x: 100, y: 100 };

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

@@ -130,6 +130,43 @@ export function zoomAtPoint(
 	};
 }
 
+/** Assumed milliseconds per animation frame, used to scale inertia velocity. */
+export const FRAME_MS = 16;
+/** Per-frame velocity decay for pan inertia (OQ-004). */
+export const INERTIA_DECAY = 0.92;
+/** Inertia stops once the velocity (svg units per ms) drops below this. */
+export const INERTIA_MIN_SPEED = 0.02;
+
+/**
+ * Pinch zoom around the gesture centroid (US-PAN-002/003). The new zoom is the
+ * start zoom scaled by the finger-distance ratio (clamped); the anchor offset
+ * keeps the centroid fixed via {@link zoomAtPoint}.
+ */
+export function pinchZoom(
+	state: PanZoomState,
+	startZoom: number,
+	startDist: number,
+	currentDist: number,
+	anchorX: number,
+	anchorY: number
+): PanZoomState {
+	const ratio = startDist > 0 ? currentDist / startDist : 1;
+	return zoomAtPoint(state, clampZoom(startZoom * ratio), anchorX, anchorY);
+}
+
+/**
+ * Advance the pan by one inertia frame: continue the release velocity (svg units
+ * per ms) in the drag direction, scaled by the frame duration. Zoom is untouched.
+ */
+export function stepInertia(
+	state: PanZoomState,
+	velX: number,
+	velY: number,
+	frameMs: number = FRAME_MS
+): PanZoomState {
+	return { x: state.x - velX * frameMs, y: state.y - velY * frameMs, z: state.z };
+}
+
 /** Linearly interpolate between two view states (drives fit/recentre tweening). */
 export function lerpView(from: PanZoomState, to: PanZoomState, t: number): PanZoomState {
 	return {

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

@@ -4,6 +4,11 @@ import {
 	clampZoom,
 	screenDeltaToSvg,
 	zoomAtPoint,
+	pinchZoom,
+	stepInertia,
+	FRAME_MS,
+	INERTIA_DECAY,
+	INERTIA_MIN_SPEED,
 	ZOOM_STEP_KB,
 	type PanZoomState
 } from '$lib/person/genealogy/panZoom';
@@ -25,8 +30,6 @@ export interface PanZoomGesturesParams {
 
 /** Pointer movement (px) below which a drag is treated as a tap, not a pan. */
 const DRAG_THRESHOLD_PX = 4;
-const INERTIA_DECAY = 0.92;
-const INERTIA_MIN_SPEED = 0.02; // svg units per ms
 
 /**
  * Touch/mouse/wheel pan & zoom for the Stammbaum canvas (#692). Thin DOM glue:
@@ -86,7 +89,7 @@ export const panZoomGestures: Action<SVGSVGElement, PanZoomGesturesParams> = (no
 		if (Math.hypot(velX, velY) < INERTIA_MIN_SPEED) return;
 		const step = () => {
 			const before = current;
-			emit({ ...current, x: current.x - velX * 16, y: current.y - velY * 16 });
+			emit(stepInertia(current, velX, velY, FRAME_MS));
 			velX *= INERTIA_DECAY;
 			velY *= INERTIA_DECAY;
 			const stalled = current.x === before.x && current.y === before.y;
@@ -135,8 +138,7 @@ export const panZoomGestures: Action<SVGSVGElement, PanZoomGesturesParams> = (no
 			const dist = distance(a, b) || 1;
 			const centroid = { x: (a.x + b.x) / 2, y: (a.y + b.y) / 2 };
 			const anchor = anchorOffset(centroid.x, centroid.y);
-			const targetZoom = clampZoom(pinchStartZoom * (dist / pinchStartDist));
-			emit(zoomAtPoint(current, targetZoom, anchor.x, anchor.y));
+			emit(pinchZoom(current, pinchStartZoom, pinchStartDist, dist, anchor.x, anchor.y));
 			moved = true;
 			return;
 		}