Add the MatrixRTC SDK package, its harness and a failing smoke test

Set up `sdk/` as a package shaped exactly like `component/`: its own
package.json with peer dependencies, the pnpm workspace marker so a host can
install it as a git dependency, a build tsconfig for type emission, and a vite
lib config with an externals list that `pnpm lint:externals` now checks for
both library builds.

The package holds only the interface for now. `createRtcSession` returns an
empty object; `RtcSession`, `RtcMember`, `MemberMedia` and `TransportMetadata`
are the contract Element Call's `CallViewModel` will be rebuilt on (see
sdk-plan.md). The shared primitives are re-exported from `src` until the first
implementation slice moves them.

`sdk/dev` is the smallest consumer of the SDK: a plain page that logs in, joins
a room and renders one tile per member, served by `pnpm dev:sdk` on port 3002.
Playwright starts it as a third web server, and `playwright/sdk/smoke.spec.ts`
has two browsers join one session. It fails on the status line with the SDK's
own error until there is an implementation behind the interface.

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

playwright/sdk/harness.ts

@@ -0,0 +1,88 @@
+/*
+Copyright 2026 Element Creations Ltd.
+
+SPDX-License-Identifier: AGPL-3.0-only OR LicenseRef-Element-Commercial
+Please see LICENSE in the repository root for full details.
+*/
+
+import { expect, type Page } from "@playwright/test";
+
+import { SynapseAdmin } from "../utils/synapse-admin.ts";
+
+/**
+ * Where the SDK harness is served — `sdk/dev`, a page that uses the SDK the way
+ * a host would, with none of Element Call in it.
+ */
+export const SDK_HARNESS_URL = "https://localhost:3002";
+
+const HOMESERVER_URL = "https://synapse.m.localhost";
+const PASSWORD = "foobarbaz1!";
+
+/**
+ * Registers two users through the Synapse admin API and has the first create a
+ * public room, so that the second can join it by id without an invite.
+ */
+export async function createUsersAndRoom(
+  name: string,
+): Promise<{ usernames: [string, string]; roomId: string }> {
+  const admin = SynapseAdmin.forHomeserver(HOMESERVER_URL);
+  const usernames: [string, string] = [
+    `${name}_a_${Date.now()}`,
+    `${name}_b_${Date.now()}`,
+  ];
+  const [{ access_token: accessToken }] = await Promise.all(
+    usernames.map(async (username, index) =>
+      admin.registerUser(username, PASSWORD, `${name} ${"AB"[index]}`),
+    ),
+  );
+
+  const response = await fetch(
+    `${HOMESERVER_URL}/_matrix/client/v3/createRoom`,
+    {
+      method: "POST",
+      headers: {
+        Authorization: `Bearer ${accessToken}`,
+        "Content-Type": "application/json",
+      },
+      body: JSON.stringify({
+        name: `${name}'s session`,
+        preset: "public_chat",
+      }),
+    },
+  );
+  if (!response.ok)
+    throw new Error(
+      `Could not create a room: ${response.status} ${await response.text()}`,
+    );
+  const { room_id: roomId } = (await response.json()) as { room_id: string };
+
+  return { usernames, roomId };
+}
+
+/**
+ * Opens the harness signed in as the given user and waits until the SDK
+ * reports the session as joined. The status line carries the SDK's error if
+ * it does not get there, so the failure says why.
+ */
+export async function startHarness(
+  page: Page,
+  username: string,
+  roomId: string,
+): Promise<void> {
+  const query = new URLSearchParams({
+    homeserver: HOMESERVER_URL,
+    username,
+    password: PASSWORD,
+    room: roomId,
+  });
+  await page.goto(`${SDK_HARNESS_URL}/?${query.toString()}`);
+  await page.getByRole("button", { name: "Start" }).click();
+
+  // A login, a crypto setup and an initial sync happen first. An error is
+  // final, so it is not worth waiting out the timeout for "Joined" after one.
+  const status = page.getByTestId("status");
+  await expect(status).toHaveText(/^(Joined|Error)/, { timeout: 120_000 });
+  const text = await status.textContent();
+  if (text !== "Joined")
+    throw new Error(`The harness did not join the session: ${text}`);
+}