- parseInt: default radix 0 (auto-detect) instead of 10, so "0xFF" works
- array_concat: use array_length_exists() to correctly spread empty arrays
- Remove unused obj_proto parameter from error constructor helpers

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Add comprehensive built-in object support for the JavaScript engine:

## Infrastructure
- NativeContext passed to native callbacks (GC access + this binding)
- Object.prototype and Array.prototype stored in VM
- CreateObject/CreateArray set prototype chains
- Array literals now set length property

## Object built-ins
- Constructor, Object.keys/values/entries/assign/create/is
- Object.getPrototypeOf/getOwnPropertyNames/getOwnPropertyDescriptor
- Object.defineProperty/freeze/seal/isFrozen/isSealed
- Object.fromEntries/hasOwn
- Object.prototype: hasOwnProperty/toString/valueOf

## Array built-ins
- Native: push/pop/shift/unshift, indexOf/lastIndexOf/includes
- Native: join/slice/concat/reverse/splice/fill/at/toString
- Static: Array.isArray/from/of
- JS preamble: map/filter/reduce/reduceRight/forEach
- JS preamble: find/findIndex/some/every/sort/flat/flatMap

## Error built-ins
- Error/TypeError/ReferenceError/SyntaxError/RangeError/URIError/EvalError
- Prototype chain with name/message, Error.prototype.toString

## Global functions
- parseInt/parseFloat/isNaN/isFinite

37 new tests covering all built-in categories.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Remaining define_local() calls in destructuring patterns, catch
parameters, and class declarations did not check captured_names,
causing closures that capture these variables to fail at runtime.
Replace all with define_local_ext() + proper cell allocation, and
remove the now-unused define_local() method.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>