Adds closeable, drainable Mailbox primitive + tests

[lePereT]

What type of PR is this? (check all applicable)

• 🍕 Feature
• ✅ Test

Description

Adds a new fibers.mailbox module: a closeable, drainable queue intended for structured-concurrency workloads where “end-of-stream” semantics are required without changing Channel behaviour.

• Introduces MailboxTx/MailboxRx with:

  • tx:send() / rx:recv() operations compatible with Op choice/abort semantics
  • explicit closure and drain behaviour (nil reserved for end-of-stream) and :why() for close reason
  • multi-producer support via tx:clone() and counted sender handles; mailbox closes when the last sender handle closes

• Adds unit tests covering:

  • rendezvous and buffered modes
  • close + drain semantics for receivers
  • rejection/wakeup semantics for blocked senders on close
  • multi-producer refcounted close behaviour and reason propagation

Manual test

• 👍 yes

Manual test description

Ran the test suite locally and exercised a simple producer/consumer example with multiple cloned senders, verifying:

• messages are delivered in order (buffered) / via rendezvous (unbuffered)
• receivers terminate with nil once closed and drained
• sends after closure are rejected and :why() reports the close reason

Added tests?

• 👍 yes

Added to documentation?

• 🙅 no documentation needed