Tree-walking ECMAScript interpreter built on ecma-syntax-cat. v0.3.0 adds accessor-property support at the engine level: `Object::with_accessor(key, AccessorPair)` installs a getter/setter pair, and `obj.key` / `obj.key = value` dispatch through the pair instead of doing plain data read/write. Embedders (web-api-cat etc.) can now wire DOM accessors like `document.cookie` via native getter/setter functions. Object-literal `{ get x() {} }` syntax remains gated on ecma-parse-cat learning to emit `ObjectPropertyKind::Get/Set` (the engine handles those AST variants already).