NTH-001: Add AST_EXTERN_DECL stub to typechecker_check_node

[Raulgooo]

Summary

typechecker_check_node has no explicit case AST_EXTERN_DECL:. Extern declarations fall through to default. Adding an explicit stub prevents silent mishandling if the default branch ever gains behavior.

Scope

Add case AST_EXTERN_DECL: break; (or a validation stub) to typechecker_check_node in src/typecheck.c.

Acceptance Criteria

• AST_EXTERN_DECL hits explicit case in check_node
• No existing tests broken

Note

Post-v0.1 polish item — not blocking any phase.