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>

sdk/README.md

@@ -0,0 +1,74 @@
+# MatrixRTC SDK (EXPERIMENTAL)
+
+`@element-hq/matrixrtc-sdk` is the call model under Element Call, on its own:
+MatrixRTC memberships and transports, LiveKit connections, publishing, E2EE keys and
+the media of every member, as observables. It has no UI. Element Call's own
+`CallViewModel` is meant to become one consumer of it; the design is in
+[`sdk-plan.md`](../sdk-plan.md).
+
+**Status:** interface only. `createRtcSession` returns an object that does nothing.
+The development harness and its e2e test exist so the implementation can be built
+against them.
+
+## Using it
+
+```ts
+import {
+  constant,
+  createRtcSession,
+  E2eeType,
+  MatrixRTCMode,
+  ObservableScope,
+} from "@element-hq/matrixrtc-sdk";
+
+const scope = new ObservableScope();
+const session = createRtcSession(
+  scope,
+  client, // a matrix-js-sdk MatrixClient, logged in and syncing
+  room, // the matrix-js-sdk Room to hold the session in
+  {
+    microphoneEnabled$: constant(true),
+    cameraEnabled$: constant(true),
+    audioInputDeviceId$: constant(undefined),
+    videoInputDeviceId$: constant(undefined),
+    videoProcessor$: constant(undefined),
+  },
+  {
+    encryptionSystem: { kind: E2eeType.PER_PARTICIPANT },
+    matrixRTCMode: MatrixRTCMode.Compatibility,
+  },
+);
+session.join();
+
+session.remoteMembers$.subscribe((members) => {
+  // each member has displayName$, media$ and more; a media track is rendered
+  // by handing it a <video> or <audio> element: track.attach(element)
+});
+
+// later
+session.leave();
+scope.end();
+```
+
+[`dev/main.ts`](./dev/main.ts) is this example as a working page.
+
+## Development
+
+Everything runs from the repository root. This directory is a pnpm project of its
+own only so that a host can install it as a git dependency (see
+`pnpm-workspace.yaml`).
+
+```sh
+pnpm dev:sdk              # the harness, https://localhost:3002
+pnpm build:sdk            # dist/matrixrtc-sdk.js and dist/types
+pnpm test:playwright playwright/sdk   # needs `pnpm backend`
+```
+
+## Installing
+
+```sh
+pnpm add github:element-hq/element-call#<ref>&path:/sdk
+```
+
+The host supplies `matrix-js-sdk`, `livekit-client` and `rxjs`; the build leaves
+them external.