feat: support decorators on type expressions

[Copilot]

Decorators were previously rejected on all type expressions. This adds a DecoratedExpression AST node that wraps any expression with decorators, enabling inline decoration of models, type references, unions, and is/extends targets.

model Pet {
  owner: @doc("Owner contact info") { name: string; phone: string };
}

alias Address = @doc("A mailing address") { street: string; city: string };

model Extended is @tag("extended") Base { extra: string }

Compiler changes

• types.ts: New SyntaxKind.DecoratedExpression, DecoratedExpressionNode interface, added to Expression union
• parser.ts: parsePrimaryExpression now calls parseDecoratedExpression() instead of reportInvalidDecorators; visitChildren updated
• checker.ts: checkDecoratedExpression checks target then applies decorators individually (avoids re-applying existing decorators on already-finished types). checkModelIs and checkClassHeritage unwrap DecoratedExpression to validate the inner target correctly
• printer.ts: printDecoratedExpression for formatter support
• ast/index.ts: Export DecoratedExpressionNode

Tests

• 6 parser tests: decorated inline models, aliases, type references, is, unions, arrays
• 6 checker tests: @doc on inline models and aliases, decorators on type references, is expressions, custom decorators, union expressions

Documentation

• decorators.md: Updated scope description; new "Decorating expressions" section
• models.md: New "Decorating inline models" section
• Documentation sample updated with @doc on an inline model expression

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