What problem does this PR solve?

Issue Number: resolve

Problem Summary:

Socket maintains _unwritten_bytes as the source of truth for pending
output, but also caches the overcrowding state in a separate volatile bool
named _overcrowded.

WriteRequest::Setup() and CancelUnwrittenBytes() can update this cached
boolean concurrently with Socket::Write() reading it. volatile does not
provide synchronization, so these accesses form a C++ data race. The cached
state can also become stale when concurrent increment and decrement paths
overwrite each other.

What is changed and the side effects?

Changed:

• Remove the _overcrowded cached boolean.
• Derive the overcrowding state from an atomic load of _unwritten_bytes.
• Use is_overcrowded() in write admission checks and socket debug output.
• Add unit coverage for the boundary behavior around
  -socket_max_unwritten_bytes.
• Document that a non-positive unwritten-byte limit disables overcrowding.
  Update the KeepWrite comment and exercise overcrowding transitions through
  the public socket write path.

Side effects:

• Performance effects:

• Breaking backward compatibility:

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.