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.config.ts

@@ -23,6 +23,7 @@ import babel from "@rolldown/plugin-babel";
 import react, { reactCompilerPreset } from "@vitejs/plugin-react";
 import { realpathSync } from "fs";
 import * as fs from "node:fs";
+import { fileURLToPath } from "node:url";
 
 export const vitePluginsConfig = ({
   mode,
@@ -161,6 +162,11 @@ export default ({
         // which Vite for some reason refuses to work with, so we point it to
         // src/index.ts instead
         "matrix-widget-api": "matrix-widget-api/src/index.ts",
+        // The SDK by its package name, resolved to its source: see the matching
+        // entry in tsconfig.json
+        "@element-hq/matrixrtc-sdk": fileURLToPath(
+          new URL("./sdk/index.ts", import.meta.url),
+        ),
       },
       dedupe: [
         "react",