SPARQL query service (#754)

SPARQL 1.1 query service wrapping pub/sub triples interface

Add a backend-agnostic SPARQL query service that parses SPARQL
queries using rdflib, decomposes them into triple pattern lookups
via the existing TriplesClient pub/sub interface, and performs
in-memory joins, filters, and projections.

Includes:
- SPARQL parser, algebra evaluator, expression evaluator, solution
  sequence operations (BGP, JOIN, OPTIONAL, UNION, FILTER, BIND,
  VALUES, GROUP BY, ORDER BY, LIMIT/OFFSET, DISTINCT, aggregates)
- FlowProcessor service with TriplesClientSpec
- Gateway dispatcher, request/response translators, API spec
- Python SDK method (FlowInstance.sparql_query)
- CLI command (tg-invoke-sparql-query)
- Tech spec (docs/tech-specs/sparql-query.md)

New unit tests for SPARQL query

trustgraph-base/trustgraph/schema/services/__init__.py

@@ -13,4 +13,5 @@ from .rows_query import *
 from .diagnosis import *
 from .collection import *
 from .storage import *
-from .tool_service import *
+from .tool_service import *
+from .sparql_query import *

trustgraph-base/trustgraph/schema/services/sparql_query.py

@@ -0,0 +1,40 @@
+from dataclasses import dataclass, field
+
+from ..core.primitives import Error, Term, Triple
+from ..core.topic import queue
+
+############################################################################
+
+# SPARQL query
+
+@dataclass
+class SparqlBinding:
+    """A single row of SPARQL SELECT results.
+    Values are ordered to match the variables list in SparqlQueryResponse.
+    """
+    values: list[Term | None] = field(default_factory=list)
+
+@dataclass
+class SparqlQueryRequest:
+    user: str = ""
+    collection: str = ""
+    query: str = ""           # SPARQL query string
+    limit: int = 10000        # Safety limit on results
+
+@dataclass
+class SparqlQueryResponse:
+    error: Error | None = None
+    query_type: str = ""      # "select", "ask", "construct", "describe"
+
+    # For SELECT queries
+    variables: list[str] = field(default_factory=list)
+    bindings: list[SparqlBinding] = field(default_factory=list)
+
+    # For ASK queries
+    ask_result: bool = False
+
+    # For CONSTRUCT/DESCRIBE queries
+    triples: list[Triple] = field(default_factory=list)
+
+sparql_query_request_queue = queue('sparql-query', cls='request')
+sparql_query_response_queue = queue('sparql-query', cls='response')