fix: stop pushing SPARQL LIMIT into child algebra nodes (#946)

The Slice evaluator was propagating the SPARQL LIMIT value as the
inner limit for child evaluations, starving LeftJoin (OPTIONAL) and
other operators of results. The safety limit parameter should flow
through unchanged; LIMIT/OFFSET are applied only at the Slice node.

trustgraph-flow/trustgraph/query/sparql/algebra.py

@@ -262,13 +262,7 @@ async def _eval_order_by(node, tc, collection, limit):
 
 async def _eval_slice(node, tc, collection, limit):
     """Evaluate a Slice node (LIMIT/OFFSET)."""
-    # Pass tighter limit downstream if possible
-    inner_limit = limit
-    if node.length is not None:
-        offset = node.start or 0
-        inner_limit = min(limit, offset + node.length)
-
-    solutions = await evaluate(node.p, tc, collection, inner_limit)
+    solutions = await evaluate(node.p, tc, collection, limit)
     return slice_solutions(solutions, node.start or 0, node.length)