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.config.ts

@@ -12,6 +12,7 @@ import path from "node:path";
 import { fileURLToPath } from "node:url";
 
 import { COMPONENT_HARNESS_URL } from "./playwright/component/harness.ts";
+import { SDK_HARNESS_URL } from "./playwright/sdk/harness.ts";
 
 const baseURL = process.env.USE_DOCKER
   ? "http://localhost:8080"
@@ -146,5 +147,17 @@ export default defineConfig({
         timeout: 500,
       },
     },
+    {
+      // The harness that uses the MatrixRTC SDK without Element Call. A Vite
+      // dev server for the same reason as the component's.
+      command: "pnpm dev:sdk",
+      url: SDK_HARNESS_URL,
+      reuseExistingServer: !process.env.CI,
+      ignoreHTTPSErrors: true,
+      gracefulShutdown: {
+        signal: "SIGTERM",
+        timeout: 500,
+      },
+    },
   ],
 });