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-dev.config.ts

@@ -0,0 +1,49 @@
+/*
+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 { defineConfig, searchForWorkspaceRoot } from "vite";
+import { realpathSync } from "node:fs";
+import * as fs from "node:fs";
+
+import { vitePluginsConfig } from "./vite.config.ts";
+
+// Serves the SDK's development harness, `sdk/dev`: a page that uses the SDK as a
+// host would, and nothing of Element Call. The shape of the component's
+// development config, without the styles.
+export default defineConfig(({ mode }) => {
+  const allow = [searchForWorkspaceRoot(process.cwd())];
+  for (const path of [
+    "node_modules/matrix-js-sdk/node_modules/@matrix-org/matrix-sdk-crypto-wasm",
+    "node_modules/@matrix-org/matrix-sdk-crypto-wasm",
+  ]) {
+    try {
+      allow.push(realpathSync(path));
+    } catch {}
+  }
+
+  return {
+    ...vitePluginsConfig({ mode, html: false }),
+    root: "sdk/dev",
+    publicDir: false,
+    server: {
+      host: true,
+      // After the app (3000) and the component harness (3001), so all three can
+      // run at once
+      port: 3002,
+      fs: { allow },
+      // The same certificate the app uses, so that the harness is served from
+      // a `m.localhost` name the development homeserver's certificate covers
+      https: {
+        key: fs.readFileSync("./backend/dev_tls_m.localhost.key"),
+        cert: fs.readFileSync("./backend/dev_tls_m.localhost.crt"),
+      },
+    },
+    worker: {
+      format: "es",
+    },
+  };
+});