Fix bthread_join memory visibility with paired release/acquire on version_butex

[chenBright]

What problem does this PR solve?

Issue Number: resolve #3274

Problem Summary:

The join/end handshake on version_butex was not correctly synchronized.

• Producer (task_runner, at bthread end): bumped the version with a plain write
  ++*m->version_butex. The surrounding version_lock provides release semantics
  only to threads that also take that lock, but join() does not, so there is no release on
  version_butex for the join path.
• Consumer (TaskGroup::join): exited its wait loop via a plain read *m->version_butex,
  with no acquire ordering.

The result is a data race with no happens-before edge, so writes the joined bthread made
before ending were not guaranteed visible after join() returned. On x86 (TSO) the hardware
masked this, but on ARM it surfaced.

PR #3276 attempted a fix by adding a lone atomic_thread_fence(acquire) after the loop. But
per the C++ memory model an acquire fence only establishes synchronization when a preceding
atomic load reads a value from a matching release operation. Here the load was a plain read and
the producer had no release store, so the fence pairs with nothing. It happens to work on ARM
only because the compiler emits a real dmb ishld, not because the model guarantees it.

What is changed and the side effects?

Changed:

This PR replaces that band-aid with a properly paired release/acquire on version_butex.

Side effects:

• Performance effects:

• Breaking backward compatibility:

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Check List:

• Please make sure your changes are compilable.
• When providing us with a new feature, it is best to add related tests.
• Please follow Contributor Covenant Code of Conduct.

[Copilot]

🟡 Changes recommended

Resolve the remaining version_butex data race and add the requested regression test.

Get a fresh assessment by requesting another Copilot review.

Pull request overview

Fixes bthread_join memory visibility by pairing release publication with acquire observation on version_butex.

Changes:

• Atomically publishes version updates at task completion.
• Uses acquire loads during joining.
• Documents synchronization semantics.

File summaries

File	Summary
src/bthread/task_meta.h	Updates version synchronization documentation.
src/bthread/task_group.cpp	Implements release/acquire handling; TaskGroup::exists() still has an unresolved plain-read race, and a visibility regression test is requested.

Review details

• Files reviewed: 2/2 changed files
• Comments generated: 2
• Review effort level: Lite

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[Copilot]

🔵 Needs a closer look

Low-level concurrency changes warrant final human review.

Review details

• Files reviewed: 4/4 changed files
• Comments generated: 0 new
• Review effort level: Lite