fix: :: TypeName on struct/enum captures names the type instead of aliasing Node

[zharinov]

Summary

• Type annotation :: Name on struct captures ({...} @x :: Name) now names the struct instead of creating a Node alias
• Same fix applies to enum captures ([A: ... B: ...] @x :: Name)
• Extracted annotation_to_alias helper to deduplicate annotation→type conversion

Why

Bug: {(fn) @fn} @outer :: FunctionInfo generated type FunctionInfo = Node instead of interface FunctionInfo { fn: Node }.

The annotation was eagerly creating a TypeShape::Custom that always aliased Node, ignoring the struct's actual fields.