refactor(web): centralize touch mouse adapters

apps/web/src/hooks/timelineTouch.test.ts

@@ -1,5 +1,5 @@
 import { describe, expect, it } from "vitest";
-import { getTouchPoint, resolveTouchDragDecision } from "./timelineTouch.js";
+import { createTouchMouseDownEvent, getTouchPoint, resolveTouchDragDecision } from "./timelineTouch.js";
 
 describe("timelineTouch", () => {
   it("falls back from active touches to changedTouches and then zeroes", () => {
@@ -72,4 +72,25 @@ describe("timelineTouch", () => {
       shouldHandleDrag: true,
     });
   });
+
+  it("creates a primary-button mouse-down adapter for touch handlers", () => {
+    const currentTarget = new EventTarget();
+
+    expect(createTouchMouseDownEvent({ clientX: 42, clientY: 17 }, currentTarget)).toMatchObject({
+      button: 0,
+      clientX: 42,
+      currentTarget,
+      shiftKey: false,
+    });
+  });
+
+  it("allows missing targets when creating a touch mouse-down adapter", () => {
+    const event = createTouchMouseDownEvent({ clientX: 7, clientY: 9 });
+
+    expect(event.currentTarget).toBeNull();
+    expect(() => {
+      event.preventDefault();
+      event.stopPropagation();
+    }).not.toThrow();
+  });
 });