Build failure with swift-nightly-main: region-isolation errors in MultiProducerSingleConsumerAsyncChannel

swift-async-algorithms 1.1.3 fails to compile with the current swift-nightly-main toolchain.

All errors are in `MultiProducerSingleConsumerAsyncChannel+Internal.swift` and look like this:

```
error: '$0.enqueueContinuation' cannot be returned
note: returning '$0.enqueueContinuation' risks concurrent access to 'inout sending' parameter '$0'
as caller assumes '$0' and result can be sent to different isolation domains
```

Same pattern repeats for `enqueueProducer`, `cancelProducer`, `finish`, `next`, `suspendNext`, and `cancelNext`.

**To reproduce:**

```bash
docker run --rm swiftlang/swift:nightly-main-jammy \
bash -cl "git clone https://github.com/apple/swift-async-algorithms.git && cd swift-async-algorithms && swift build"
```

Looks like the nightly compiler tightened region-isolation checks around values returned from `Mutex.withLock` closures that use `inout sending` parameters.

Start with MultiProducerSingleConsumerAsyncChannel+Internal.swift and reproduce the failure using the provided swiftlang/swift:nightly-main-jammy Docker command. Inspect the Mutex.withLock closures around enqueueContinuation, enqueueProducer, cancelProducer, finish, next, suspendNext, and cancelNext. Done means swift build succeeds with the current nightly-main toolchain.

Written by the indexing model from the issue text.