Move the SDK demo to sdk-target-based-on-call-view-model

The `sdk/` directory is about to become the MatrixRTC SDK package, so the
demo that lives there — one bundle of Element Call's CallViewModel for a plain
page or widget — moves aside, unchanged. It keeps building under a script and
vite config of its own, `pnpm build:sdk-target-based-on-call-view-model`, into
its own `dist` rather than the repository root's, and no longer copies the
developer's config.json out of `public`. Its README says which script to run
and where the output lands.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

sdk-target-based-on-call-view-model/helper.ts

@@ -0,0 +1,51 @@
+/*
+Copyright 2025 Element Creations Ltd.
+
+SPDX-License-Identifier: AGPL-3.0-only OR LicenseRef-Element-Commercial
+Please see LICENSE in the repository root for full details.
+*/
+
+/**
+ * This file contains helper functions and types for the MatrixRTC SDK.
+ */
+
+import { logger as rootLogger } from "matrix-js-sdk/lib/logger";
+import { scan } from "rxjs";
+
+import { type WidgetHelpers } from "../src/widget";
+import { type LivekitRoomItem } from "../src/state/CallViewModel/CallViewModel";
+
+export const tryMakeSticky = (widget: WidgetHelpers): void => {
+  const logger = rootLogger.getChild("[MatrixRTCSdk]");
+  logger.info("try making sticky MatrixRTCSdk");
+  void widget.api
+    .setAlwaysOnScreen(true)
+    .then(() => {
+      logger.info("sticky MatrixRTCSdk");
+    })
+    .catch((error) => {
+      logger.error("failed to make sticky MatrixRTCSdk", error);
+    });
+};
+export const TEXT_LK_TOPIC = "matrixRTC";
+/**
+ * simple helper operator to combine the last emitted and the current emitted value of a rxjs observable
+ *
+ * I think there should be a builtin for this but i did not find it...
+ */
+export const currentAndPrev = scan<
+  LivekitRoomItem[],
+  {
+    prev: LivekitRoomItem[];
+    current: LivekitRoomItem[];
+  }
+>(
+  ({ current: lastCurrentVal }, items) => ({
+    prev: lastCurrentVal,
+    current: items,
+  }),
+  {
+    prev: [],
+    current: [],
+  },
+);