chore: add changeset for the livekit-rpc extraction

Author: 1egoman

.changeset/extract_livekit_rpc_crate.md

@@ -0,0 +1,35 @@
+---
+livekit-rpc: patch
+livekit: patch
+livekit-ffi: patch
+---
+
+Moves the RPC implementation into a new `livekit-rpc` crate, alongside the existing
+`livekit-data-stream` and `livekit-datatrack` crates.
+
+This is fully backwards compatible: `livekit` re-exports the crate at the historical
+`livekit::rpc` and `livekit::participant` paths, and the prelude still provides
+`PerformRpcData`, `RpcError`, `RpcErrorCode` and `RpcInvocationData`. Within the `livekit`
+crate itself, RPC types are now imported from `livekit-rpc` directly rather than through
+those re-exports. `RpcClientManager`, `RpcServerManager` and `HandleRequestOptions` remain
+reachable but are now `#[doc(hidden)]`: they are internal SDK API and were never usable
+without the (private) transport trait.
+
+The new crate does not depend on `libwebrtc`, so its unit tests run without building WebRTC.
+The transport seam that made this possible was already in place; the only change to it is
+that `RpcTransport::publish_data` now returns a message-only `RpcTransportError` instead of
+`livekit::RoomError`, mirroring `livekit_data_stream::api::SendError`.
+
+Also fixes four latent bugs found while moving the code:
+
+- An RPC call to a participant who disconnects mid-call now fails promptly with
+  `RecipientDisconnected`. Pending calls were never purged on disconnect, so the caller
+  waited out its full response timeout (15s by default) and got `ResponseTimeout` instead.
+- A server reporting a version that is not valid semver no longer panics the calling task.
+  An unparseable version is no longer treated as evidence that the server is too old.
+- A v1 `RpcResponse` carrying a compressed payload, or no value at all, now fails with an
+  `ApplicationError` instead of resolving the caller with an empty successful response.
+- Removed an unguarded `unwrap` when building a v1 response packet, by giving the function
+  a signature that cannot represent the invalid state.
+
+Also drops the `semver` dependency from `livekit`, which was only used by the RPC client.