[Query] Respect filter scope in nested group graph patterns (dawg-optional-filter-005-simplified)

[remiceres]

Summary

Address the remaining SPARQL 1.0 test failure dawg-optional-filter-005-simplified in corese-next.

Targeted W3C Test Case

• Suite: SPARQL 1.0 Query Evaluation
• Test ID: dawg-optional-filter-005-simplified
• Description: Double curly braces get simplified to single curly braces early on, before filters are scoped.

Context

In standard SPARQL 1.0/1.1 algebra translation (W3C Query §18.2.2.6), a GroupGraphPattern with nested braces { { P } OPTIONAL { ... } } introduces an inner scope. Filters defined inside the inner group pattern must not be floated out or merged before the scoping boundary is established.

Acceptance Criteria

• Nested group graph pattern scoping preserves filter boundaries before simplification.
• dawg-optional-filter-005-simplified passes in corese-w3c.
• 0 regressions across existing W3C suites.