feat: dependent types — refinement types, type aliases, type annotations
- Lexer: added 'type' and 'where' keywords
- AST: TypeExpr::Refined, Declaration::TypeAlias, LetDecl.type_annotation
- Parser: parse_type_alias_decl, parse_type_expr (Named, Generic, where)
- Type system: Type::Refined, Predicate/PredicateExpr with evaluate_static()
- Errors: RefinementViolation, TypeAliasCycle (Elm-style messages)
- Checker: type alias registry, resolve_type_expr, ast_to_predicate,
static refinement checking for literal values
- Codegen: Phase 1b runtime guards, predicate_to_js helper
Syntax: type PositiveInt = Int where value > 0
let count: PositiveInt = 5 -- static check passes
let count: PositiveInt = -1 -- compile error
118 tests pass (16 in ds-types, 5 new for refinements)
9ef28bb53a