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/lexer.rs

@@ -40,6 +40,9 @@ pub enum TokenKind {
     As,
     From,   // from (for migrations)
     Latest, // latest (for @latest version constraint)
+    Trait,  // trait (for type classes)
+    Impl,   // impl (for trait implementations)
+    For,    // for (in impl Trait for Type)
 
     // Behavioral type keywords
     Is,            // is (for behavioral properties)
@@ -118,6 +121,9 @@ impl fmt::Display for TokenKind {
             TokenKind::As => write!(f, "as"),
             TokenKind::From => write!(f, "from"),
             TokenKind::Latest => write!(f, "latest"),
+            TokenKind::Trait => write!(f, "trait"),
+            TokenKind::Impl => write!(f, "impl"),
+            TokenKind::For => write!(f, "for"),
             TokenKind::Is => write!(f, "is"),
             TokenKind::Pure => write!(f, "pure"),
             TokenKind::Total => write!(f, "total"),
@@ -550,6 +556,9 @@ impl<'a> Lexer<'a> {
             "as" => TokenKind::As,
             "from" => TokenKind::From,
             "latest" => TokenKind::Latest,
+            "trait" => TokenKind::Trait,
+            "impl" => TokenKind::Impl,
+            "for" => TokenKind::For,
             "is" => TokenKind::Is,
             "pure" => TokenKind::Pure,
             "total" => TokenKind::Total,