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.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 {