fix(RadialKnob): real pointer capture via setPointerCapture interop

Switch initiator to @onpointerdown; capture the pointer on the knob element
through a new knob.ts helper so pointermove/up/cancel reach the knob even
when the cursor leaves the window. Accurate comment; IAsyncDisposable cleanup.

DeepDrftShared.Client/Interop/knob/knob.ts

@@ -0,0 +1,20 @@
+/**
+ * knob - pointer capture helpers for RadialKnob.
+ *
+ * setPointerCapture / releasePointerCapture are not exposed via Blazor's
+ * ElementReference, so the component delegates here via JS interop.
+ * Both functions are no-ops when the element reference is stale (e.g. the
+ * component was disposed between the JS call and the microtask).
+ */
+
+/** Capture the pointer on the given element so pointermove/pointerup are
+ *  delivered even when the cursor leaves the browser window. */
+export function capturePointer(el: Element, pointerId: number): void {
+    (el as HTMLElement).setPointerCapture(pointerId);
+}
+
+/** Release a previously captured pointer.  Called on pointercancel.
+ *  pointerup releases capture implicitly, but we call this on cancel too. */
+export function releasePointer(el: Element, pointerId: number): void {
+    (el as HTMLElement).releasePointerCapture(pointerId);
+}