fix(spf): resolve typecheck errors in machine.ts and track-playback-initiated test

Use function form of `update()` in `createMachineCore` transition to
satisfy generic `Partial<Snapshot>` constraint. Omit `url` property
instead of setting it to `undefined` to satisfy `exactOptionalPropertyTypes`.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: cjpillsbury

packages/spf/src/core/machine.ts

@@ -42,6 +42,6 @@ export function createMachineCore<FullState extends string, Snapshot extends Mac
 ) {
   const snapshotSignal = signal(initialSnapshot);
   const getState = (): FullState => untrack(() => snapshotSignal.get().value);
-  const transition = (to: FullState): void => update(snapshotSignal, { value: to });
+  const transition = (to: FullState): void => update(snapshotSignal, (current) => ({ ...current, value: to }));
   return { snapshotSignal, getState, transition };
 }

packages/spf/src/dom/features/tests/track-playback-initiated.test.ts

@@ -129,7 +129,7 @@ describe('trackPlaybackInitiated', () => {
     await new Promise((resolve) => setTimeout(resolve, 10));
     expect(state.get().playbackInitiated).toBe(true);
 
-    state.set({ ...state.get(), presentation: { url: undefined } });
+    state.set({ ...state.get(), presentation: {} });
     await new Promise((resolve) => setTimeout(resolve, 20));
 
     expect(state.get().playbackInitiated).toBe(false);