> Any discussion which version of smw is
expected to have this common library
There is no discussion about this, as quite some work needs to happen before we can use Ask in SMW. All I can provide is a guess for the first release in which SMW might realistically be using this library, which is SMW 1.11. That is all assuming we manage to stick to our release roadmap this time.
> I assume it's the same syntax
as before, minus subobjects?
This library is a set of entities that represent data and contain some business logic. It does not define syntax. When SMW adopts this library, it can retain its current wikitext parsing and formatting code. Though I hope this code will have been rewritten by that point (without functional changes), as it is very much legacy code as well.
A library build on top of this new one provides the serialization and deserialization code for the Ask language that will be used by the Wikidata project. This could be used in SMWs web API instead, or as addition to, the wikitext serialization of Ask. A release announcement of this will follow hopefully soon.