Cover composite-key Row2 path-keyed @sourceRow classification

R110 shipped @sourceRow with Row2..Row22 arity admitted by the resolver: the per-position type loop in SourceRowDirectiveResolver iterates the lifter’s RowN type arguments without special-casing arity 1, and the leaf-PK arm constructs LifterLeafKeyed over whatever the leaf’s TableRef.primaryKeyColumns() returns. The existing SourceRowClassificationCase test enum exercises Row2 only on the rejection path (LEAF_PK_ARITY_MISMATCH against inventory.inventory_id); no successful Row2 path-keyed classification fires anywhere in the test corpus today. The gap is in the test catalog (no 2-column FK exists in graphitron-rewrite/graphitron/src/test/…​), not in resolver / emitter code.

This item adds (a) a 2-column FK to the test catalog (or selects an existing 2-column FK from Sakila when extending the sakila-example fixture), (b) a COMPOSITE_KEY_ROW2_PATH_KEYED case in SourceRowClassificationCase that asserts a Row2<…​>-returning lifter classifies as RecordTableField with a LifterPathKeyed whose parentSideColumns() matches the FK’s two-column source-side, and (c) a pipeline test exercising the parent VALUES emission and JOIN ON predicate render for the 2-column shape. Architect-review feedback from R110’s In Review pass surfaced this as a real coverage gap that scopes outside R110 itself.