A C# `delegate` is a named type, but `delegate_declaration` was in neither
`class_types` nor `function_types`, so the declaration was dropped from the
graph. A field or parameter typed as the delegate (`public BinOp Op;`,
`int Apply(BinOp f, ...)`) still resolved the name `BinOp` through
`ensure_named_node`, which minted a *sourceless* stub and pointed the
`references` edge at it — even though `BinOp` is declared in the same file.
That violates the provenance invariant: a real source-backed declaration must
not lose to a sourceless god node (and the bare-id stub becomes a cross-file
hub shared by every same-named reference in the corpus).
`delegate_declaration` has a `name` field and no body, so adding it to the C#
`class_types` emits a clean empty type node (no phantom members) that the
in-file references now bind to. Namespaced delegates are scoped and contained
exactly like the sibling class/enum/struct declarations.
Delegate parameter/return type references and delegate-instance invocation
resolution are unchanged (out of scope).
