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>

tsconfig.json

@@ -20,6 +20,10 @@
     "useDefineForClassFields": false,
     "allowImportingTsExtensions": true,
     "paths": {
+      // Element Call imports the SDK by its package name, so that the move to a
+      // published dependency changes nothing but this line (and the matching
+      // alias in vite.config.ts)
+      "@element-hq/matrixrtc-sdk": ["./sdk/index.ts"],
       // These imports within @livekit/components-core and
       // @livekit/components-react are broken under the "bundler" module
       // resolution mode, so we need to resolve them manually
@@ -53,6 +57,7 @@
     "./src/**/*.ts",
     "./src/**/*.tsx",
     "./playwright/**/*.ts",
+    "./sdk/**/*.ts",
     "./sdk-target-based-on-call-view-model/**/*.ts",
     "./component/**/*.ts",
     "./component/**/*.tsx"