feat(selfhost): @[Checked] borrow checker — basic write-through-ptr detection

- Add isChecked field to Decl struct (ast.bux)
- Parse @[Checked] attribute in parserParseDecl (parser.bux)
- Add checkedFunc flag and movedVars to Sema struct (sema.bux)
- Enable borrow checking per-function in Sema_Analyze
- Reject assignment through raw pointer (*T) in @[Checked] functions
- Skip newlines between @[Checked] attribute and the declaration keyword
  to avoid parser seeing the attribute and function as separate decls

src/ast.bux

@@ -250,6 +250,7 @@ struct Decl {
     column: uint32,
     isPublic: bool,
     isAsync: bool,
+    isChecked: int,          // @[Checked] attribute (0/1)
     // Names
     strValue: String,        // decl name
     strValue2: String,       // interface name, dll name, module path