Put the media behind a backend seam and move LiveKit into it

The session speaks MatrixRTC and sees media through MediaBackend in
sdk/src/backend/api.ts. Everything LiveKit lives in
sdk/src/backend/livekit/,
the only place in the SDK that imports livekit-client; the import
boundary rule
now enforces that outside sdk/src/backend/ the LiveKit packages appear
in type
imports only.

The seam sits above the connection manager: one connection per transport
url
is the LiveKit multi-SFU strategy, not a MatrixRTC concept, so a
cascading SFU
or full mesh replaces the folder, not the session. A backend prepares
the local
transport the session discovered (the token exchange and the delegation
probe,
reported as canDelegateDelayedLeave), hands the delayed leave over per
delay
id, gives the local member one publish switch plus publish/unpublish,
builds a
remote member's media from its membership and registers that
membership's
transport for as long as the member lives, and reports its connections
for the
transport registry, which keys on the raw transport serialised with
sorted
keys. The backend's context carries no Matrix client (a backend picks
what it
calls in its own options), no membership list, and no js-sdk session:
media
keys arrive as a stream replayed on subscribe, and MatrixKeyProvider
subscribes
to it. The MatrixRTC mode no longer reaches LiveKit; it becomes a
tokenEndpoint
choice made where the default backend is built.

Two behaviour changes ride along. Publishing is one boolean, joined and
homeserver reachable, where before a sync hiccup after leave() resumed
the
upstream of tracks that were meant to stay silent. And
ResolvedConnection keeps
one ResolvedTransport object per fetch, so a state change no longer
reads as a
change of what was fetched.

MatrixRTCClientOptions gains backend?: MediaBackendFactory, defaulting
to
createLivekitBackend built from the client options.
authenticateWithTransport
takes tokenEndpoint instead of matrixRTCMode.

sdk/SdkMigration.md

@@ -8,13 +8,14 @@ symbols, so it ages with the code; the architecture document should not.
 
 ## Status
 
-| Slice                                                                                                                                                                           | State                |
-| ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | -------------------- |
-| 1. Scaffold: `sdk/` packaging, the interface, the harness, the smoke test                                                                                                       | done, October 2026   |
-| 2. First implementation of `createMatrixRTCClient` under `sdk/src`, the media adapter, the e2e suite                                                                            | done, 1 October 2026 |
-| 3. Copy the shared modules into `sdk/src/` with their tests, drop every `src/` import, turn on the import boundary                                                              | done, 2 October 2026 |
-| 4. `createCallViewModel$` consumes the client; media view models take `media$`; Element Call imports the primitives and enums from the SDK and deletes its own copies           | done, 2 October 2026 |
-| 5. `sendData`/`data$` on the client; the legacy bundle `sdk-target-based-on-call-view-model/` is rebuilt on the SDK with its interface unchanged, since it is in production use | done, 2 October 2026 |
+| Slice                                                                                                                                                                                        | State                |
+| -------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | -------------------- |
+| 1. Scaffold: `sdk/` packaging, the interface, the harness, the smoke test                                                                                                                    | done, October 2026   |
+| 2. First implementation of `createMatrixRTCClient` under `sdk/src`, the media adapter, the e2e suite                                                                                         | done, 1 October 2026 |
+| 3. Copy the shared modules into `sdk/src/` with their tests, drop every `src/` import, turn on the import boundary                                                                           | done, 2 October 2026 |
+| 4. `createCallViewModel$` consumes the client; media view models take `media$`; Element Call imports the primitives and enums from the SDK and deletes its own copies                        | done, 2 October 2026 |
+| 5. `sendData`/`data$` on the client; the legacy bundle `sdk-target-based-on-call-view-model/` is rebuilt on the SDK with its interface unchanged, since it is in production use              | done, 2 October 2026 |
+| 12. The media backend seam: everything LiveKit behind `MediaBackend` in `sdk/src/backend/livekit/`, the `backend` option, the lint guard; see [`MediaBackendPlan.md`](./MediaBackendPlan.md) | done, 4 October 2026 |
 
 Element Call now consumes the SDK. `InCallView` creates the client from the
 configuration, settings and URL parameters, and hands it to `createCallViewModel$`,