- Tokenizer/Parser: async def, async for, async with, await keywords
- AST nodes: AwaitExpression, AsyncForStatement, AsyncWithStatement
- Runtime: PythonCoroutine, CoroutineWrapper types
- Code generation: coroutines reuse generator state machine via
  yield from desugaring (await → yield from expr.__await__())
- Fix GeneratorRewriter VisitExtension to reduce one level at a time,
  preventing "must be reducible node" with DebugInfoRemovalExpression

Verified against CPython 3.14: 20/20 comparison tests identical.

23 tests covering async def, await, async with, async for,
coroutine properties, __await__ protocol, custom awaitables,
break/continue/else, nested loops, and combined patterns.

…numerable, CancelledError

- Add TaskAwaitable/ValueTaskAwaitable wrappers enabling `await` on
  Task, Task<T>, ValueTask and ValueTask<T> from Python async code
- Add AsyncEnumerableWrapper enabling `async for` over IAsyncEnumerable<T>
- Map OperationCanceledException to new CancelledError Python exception
- Add __await__, __aiter__, __anext__ resolvers in PythonTypeInfo
- Add bridge methods in InstanceOps for the resolver pattern
- ValueTask/IAsyncEnumerable support gated behind #if NET (requires .NET Core)
- Handle Task<VoidTaskResult> (internal type arg) by falling back to
  non-generic TaskAwaitable via IsVisible check

- Add 'await' keyword to generate_ops.py kwlist
- Add CancelledError factory-only exception to generate_exceptions.py
- Regenerate TokenKind, Tokenizer, PythonWalker, PythonNameBinder
- Fix CancelledError placement in ToPythonHelper to match generator order

Add CancelledError to exception_hierarchy.txt so test_pep352
test_inheritance accounts for the new builtin exception.

Isolate test_async in a separate process to prevent it from
loading IronPythonTest assembly which causes duplicate
SpecialName GetBoundMember on XmlElement in test_attrinjector.

The runtime type of async Task<T> is often a subclass like
AsyncStateMachineBox<TResult, TStateMachine>, not Task<T> itself.
Walk up the BaseType chain to find Task<T> so that await on real
async .NET operations (e.g. HttpClient.GetStringAsync) correctly
returns the result instead of None.

Instead of blocking the thread with GetAwaiter().GetResult(),
TaskAwaitable.__next__ now yields the Task back to the runner
when it's not yet completed. The runner can then wait on the Task
and resume the coroutine, enabling true concurrency between coroutines.

Allows C# code to directly await IronPython coroutines:
  object result = await coroutine;