Add async/await support to the JavaScript engine:

- Add `Await` opcode (0x82) to bytecode format
- Add `is_async` flag to bytecode Function struct
- Compile async function declarations, expressions, arrows, and methods
- Compile `await expr` to emit Await opcode
- Parse `for await...of` syntax
- VM creates internal generator for async functions, driven by promises
- Await suspends generator and sets up promise reactions for resume
- Rejected await throws into generator (preserving try/catch)
- Save/restore exception handlers across yield/await suspension points
- Async generators return async iterator wrappers
- 16 unit tests covering all async patterns

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

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

- Add Yield and Spread bytecode opcodes, is_generator flag on Function
- Implement generator functions: function*, yield, yield*, next/return/throw
- Generator objects with suspend/resume via saved registers and IP
- Compile for...of loops using the iterator protocol (@@iterator, .next())
- Add @@iterator to built-in types: Array, String, Map, Set
- Array.prototype.keys/values/entries returning proper iterators
- String.prototype[@@iterator] iterating over characters
- Fix array/object destructuring register allocation (LIFO compliance)
- Add rest element support for array destructuring ([a, ...rest])
- Add rest property support for object destructuring ({a, ...rest})
- Implement spread operator in array literals ([...arr])
- Spread opcode iterates via @@iterator protocol
- Custom iterables work with for...of
- Generator-based iterables work with for...of
- Remove 'generators' from test262 unsupported features list
- 25 new tests covering all features

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