[petralithic]

Summary

LoroWebsocketClient::join_loro fails as soon as the receiver answers JoinError(VersionUnknown). join_with_adaptor and the TS client both rejoin with an empty version in that case. So a plain Loro doc cannot rejoin a receiver that no longer recognizes its version, for example after the receiver pruned or reset its history, while the same doc joined through LoroDocAdaptor can.

This change makes join_loro resend the JoinRequest once with an empty version when it gets VersionUnknown. If the empty version is also rejected, the join fails as before. Other join errors are unchanged, and so is the wire format.

This covers the first half of #16. It does not pass receiver_version to get_alternative_version, because that would change the public CrdtDocAdaptor trait. Sending the receiver's own version back as the join version would also be wrong: the receiver would then skip history the client is missing. I'm happy to follow up if you want a specific API for that.

Tests

New rust/loro-websocket-client/tests/join_version_unknown.rs runs a small in-process WebSocket receiver:

• It rejects the client's version with VersionUnknown, then accepts an empty version and sends its history. The join succeeds and the server text is imported next to the local edit. This test fails on main with join error: VersionUnknown.
• It rejects every version. The client sends exactly two joins (its version, then an empty one) and returns an error. On main it sends only one.

cargo test --workspace in rust/ passes. cargo clippy -p loro-websocket-client --all-targets reports nothing in the changed code. I ran rustfmt only on the new test file, because cargo fmt --check already reports diffs on main in untouched files.

This is independent of #67 and #68: it only touches join_loro.