Bound GRAPH ?g returns nothing when ?g is a late-materialized EncodedSid (EXISTS/correlated contexts)

[aaj3f]

extract_graph_iri_from_binding (graph.rs:157-172) has no EncodedSid/EncodedLit arm, so a scan-sourced ref-valued ?g silently matches nothing while string-valued rows survive (verified probe). Manifests inside FILTER EXISTS { GRAPH ?g {...} } and correlated shapes; top-level scan-bound ?g works today, so the bug hides. Roadmap PR-G1 (BUG-5) — the regression test must use the EXISTS/late-materialized shape.

Found and verified by the W3C SPARQL burn-down audit (PR #1437; docs/audit/burn-down/ROADMAP.md §5 on branch test/sparql-testsuite-full-coverage).