[shsms]

BroadcastChannel (new in 1.12) had several gaps in how it closes itself, and producers had no way to tell whether anyone was still listening. This branch fixes the gaps and adds BroadcastSender.has_receivers and BroadcastReceiver.has_senders.

Changes

• Closing a receiver now wakes a task that waits on it. Receivers wait on an Event that is replaced after each wake-up, instead of an asyncio.Condition.
• Garbage-collected senders now count as closed (through weakref.finalize). The finalizer can run in any thread, or in the middle of an await, so it only updates the channel's state and wakes the receivers through call_soon_threadsafe(). aclose() still wakes them directly. Please look closely at _remove_sender(), _wake_receivers_soon(), BroadcastReceiver.ready() and their tests in tests/test_broadcast.py.
• Garbage-collected receivers detach at once. The channel still closes on the next send() without receivers, as in 1.12, so "close the initial receiver, subscribe later" keeps working.
• clone() refuses a closed sender or channel. subscribe() uses the limit and warn_on_overflow the channel was created with as its defaults.
• BroadcastChannel no longer emits Broadcast's deprecation warning. The implementation moved into a private _Broadcast class. The Broadcast docstring moved with it, so git diff --color-moved shows the real changes.
• Docs: the user guide, README, package overview and all docstring examples use BroadcastChannel, there is a new Oneshot page, and several false claims about closing are fixed.

Breaking

• Garbage-collected broadcast senders count as closed: an auto-closing channel closes when its last sender is dropped, and sender_count no longer counts dropped senders.
• BroadcastSender.clone() raises SenderClosedError or SenderError on a closed sender or channel.
• BroadcastSender.subscribe() takes its limit and warn_on_overflow defaults from the channel.
• For errors from a BroadcastChannel caused by a ChannelClosedError, isinstance(error.__cause__.channel, Broadcast) is now False.

[Marenz]

🤖 LGTM. This fixes real bugs, and the tests cover the hard cases (GC in another thread, closed loops, double detach). The release notes are accurate. ci_checks_max and pytest_min pass locally (152 tests).

Nits:

1. The PR description says "The channel still closes on the next send() without receivers, as in 1.12". The code does the opposite: send() raises NoReceiversError and keeps the channel open (_broadcast.py:499), as commit 8278c97 and the release notes say. Please fix the description and add this to the "Breaking" list (before, that send closed the channel and raised SenderError with a ChannelClosedError cause).
2. BroadcastReceiver.close() wakes receivers directly (_broadcast.py:756), while the sender finalizer uses call_soon_threadsafe(). With asyncio debug off, calling close() from another thread leaves a waiting task stuck without any error; the thread test only covers debug mode. A docstring note is probably enough, or route it through _wake_receivers_soon().
3. _sender_count -= 1 (_broadcast.py:182) runs in the finalizer, which can be any thread, and is not atomic. Worth a comment that only the wake-up is thread-safe.
4. _Broadcast is private, so there is no public type left to check ChannelClosedError.channel against. Maybe say so in the release notes.