SPARQL engine: streaming evaluation, bind joins, and expression fixes

[cybermaggedon]

Convert the SPARQL algebra evaluator from eager list-based evaluation to lazy async generators so results stream incrementally. This lets Slice terminate early (via generator cleanup) and avoids materialising full result sets for streamable operators like Project, Filter, Union, and Extend. Blocking operators (Join, LeftJoin, OrderBy, Group) materialise at their boundary then yield.

Add bind join optimization for Join nodes where one side is small (VALUES/ToMultiSet): instead of materialising both sides independently and hash-joining, iterate the small side's bindings and evaluate the large side with those bindings pre-seeded. This turns wildcard BGP queries into selective ones — e.g. VALUES ?x { } joined with a BGP now queries the triple store with ?x bound rather than fetching all triples.

Add TriplesClient.query_gen() async generator that wraps the existing streaming callback API via an asyncio.Queue bridge, yielding individual Triple objects as batches arrive.

Add streaming request path in the SPARQL query service that batches solutions from the live async generator and sends them as they fill.

Fix FILTER IN/NOT IN: rdflib represents these as RelationalExpression nodes with op="IN", not as Builtin_IN — handle both representations.

Fix Builtin_IN/Builtin_NOTIN dispatch ordering so the specific handlers are checked before the generic Builtin_ prefix match.

Fix VALUES handling for rdflib's two representations: positional (var/value) and dict-based (res).

[github-actions]

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