feat: add extern let declarations for JS FFI

Add support for `extern let name: Type` and `extern let name: Type = "jsName"`
syntax for declaring external JavaScript values. This follows the same pattern
as extern fn across all compiler passes: parser, typechecker, interpreter
(runtime error placeholder), JS backend (emits JS name directly without
mangling), formatter, linter, modules, and symbol table.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/ast.rs

@@ -223,6 +223,8 @@ pub enum Declaration {
     Impl(ImplDecl),
     /// Extern function declaration (FFI): extern fn name(params): ReturnType
     ExternFn(ExternFnDecl),
+    /// Extern let declaration (FFI): extern let name: Type
+    ExternLet(ExternLetDecl),
 }
 
 /// Function declaration
@@ -445,6 +447,19 @@ pub struct ExternFnDecl {
     pub span: Span,
 }
 
+/// Extern let declaration (FFI)
+#[derive(Debug, Clone)]
+pub struct ExternLetDecl {
+    pub visibility: Visibility,
+    /// Documentation comment
+    pub doc: Option<String>,
+    pub name: Ident,
+    pub typ: TypeExpr,
+    /// Optional JS name override: extern let foo: T = "window.foo"
+    pub js_name: Option<String>,
+    pub span: Span,
+}
+
 /// Type expressions
 #[derive(Debug, Clone, PartialEq, Eq)]
 pub enum TypeExpr {