feat: Add iterator traits, variadic calls (0-8 args), and promise cancellation

- Add ZrtlIterable and ZrtlIterator traits to zyntax_embed
  - ZyntaxArrayIterator for typed array iteration
  - ZyntaxStringCharsIterator for UTF-8 codepoint iteration
  - ZyntaxStringBytesIterator for byte iteration
  - ZrtlRangeIterator for integer ranges
  - StdIteratorAdapter/ZrtlIteratorAdapter for std::iter interop

- Extend function calls to support 0-8 arguments
  - Add call_5, call_6, call_7, call_8 functions
  - Update call_with_signature and call_native_with_signature

- Add promise cancellation support
  - Add PromiseState::Cancelled variant
  - Add cancel() and is_cancelled() methods to ZyntaxPromise
  - Update all match statements for Cancelled state
  - Cancelled state propagates through combinators

- Add INTEGRATION_PLAN.md documenting SDK status

Author: darmie