SchemaStructure.swift was the last source file at the 400-line cap. The
cos-element-consistent check (a complex type's content model may not declare two
same-named elements with different types) is a self-contained constraint with its
own helpers (collectElementDecls, elementTypeKey) and the contentModelContainers
set used only by it.
Move that cluster into SchemaElementDeclarationsConsistent.swift;
elementDeclsConsistentErrors becomes internal (its sole caller is in
SchemaStructure). Pure code move, no behaviour change: SchemaStructure 400 -> 342,
new file 59. With this, no source file remains at the 400-line cap. Full suite and
the XSTS gate pass unchanged.
