feat: implement type classes / traits

Add support for type classes (traits) with full parsing, type checking, and
validation. The implementation includes:

- Trait declarations: trait Show { fn show(x: T): String }
- Trait implementations: impl Show for Int { fn show(x: Int) = ... }
- Super traits: trait Ord: Eq { ... }
- Trait constraints in where clauses: where T: Show + Eq
- Type parameters on traits: trait Functor<F> { ... }
- Default method implementations
- Validation of required method implementations

This provides a foundation for ad-hoc polymorphism and enables
more expressive type-safe abstractions.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/modules.rs

@@ -49,8 +49,9 @@ impl Module {
                     Declaration::Function(f) => f.visibility == Visibility::Public,
                     Declaration::Let(l) => l.visibility == Visibility::Public,
                     Declaration::Type(t) => t.visibility == Visibility::Public,
-                    // Effects and handlers are always public for now
-                    Declaration::Effect(_) | Declaration::Handler(_) => true,
+                    Declaration::Trait(t) => t.visibility == Visibility::Public,
+                    // Effects, handlers, and impls are always public for now
+                    Declaration::Effect(_) | Declaration::Handler(_) | Declaration::Impl(_) => true,
                 }
             })
             .collect()