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>

vite-sdk.config.ts

@@ -0,0 +1,65 @@
+/*
+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.
+*/
+
+import { defineConfig } from "vite";
+
+import { vitePluginsConfig } from "./vite.config.ts";
+
+// Config for the MatrixRTC SDK, a library a host imports rather than a page.
+// The shape of the component build (vite-component.config.ts) without
+// anything to do with styles or React.
+export default defineConfig(({ mode }) => {
+  const base = vitePluginsConfig({ mode, html: false });
+  return {
+    ...base,
+    // A library has no public directory to serve
+    publicDir: false,
+    build: {
+      // Into the package directory, so that `sdk/package.json` describes what
+      // sits next to it and the directory can be installed as a package
+      outDir: "sdk/dist",
+      minify: mode === "production",
+      sourcemap: true,
+      lib: {
+        formats: ["es" as const],
+        entry: { "matrixrtc-sdk": "./sdk/index.ts" },
+        fileName: (_format, entryName) => `${entryName}.js`,
+      },
+      rollupOptions: {
+        // The host already has these, and a second copy of any of them is worse
+        // than dead weight: the Matrix client would run two sync loops, and a
+        // second LiveKit or RxJS would not share state with the host's. Every
+        // subpath has to be named (see vite-component.config.ts for why), and
+        // `pnpm lint:externals` fails if the source imports one this list
+        // misses.
+        external: [
+          "livekit-client",
+          "matrix-js-sdk",
+          "matrix-js-sdk/lib/browser-index",
+          "matrix-js-sdk/lib/client",
+          "matrix-js-sdk/lib/crypto-api",
+          "matrix-js-sdk/lib/indexeddb-worker",
+          "matrix-js-sdk/lib/logger",
+          "matrix-js-sdk/lib/matrix",
+          "matrix-js-sdk/lib/matrixrtc",
+          "matrix-js-sdk/lib/matrixrtc/EncryptionManager",
+          "matrix-js-sdk/lib/matrixrtc/IKeyTransport",
+          "matrix-js-sdk/lib/matrixrtc/IMembershipManager",
+          "matrix-js-sdk/lib/models/relations-container",
+          "matrix-js-sdk/lib/models/room",
+          "matrix-js-sdk/lib/models/typed-event-emitter",
+          "matrix-js-sdk/lib/randomstring",
+          "matrix-js-sdk/lib/sync",
+          "matrix-js-sdk/lib/types",
+          "matrix-js-sdk/lib/utils",
+          "rxjs",
+          "rxjs/testing",
+        ],
+      },
+    },
+  };
+});