Implement stop token in ChannelRaw for improved control

[planetis-m]

Summary

This PR makes Chan safer to shut down and easier to use in real-world threaded systems:

• Adds a channel stop token (Atomic[bool]) and public control APIs:
  • chan.stop() to request shutdown and wake blocked waiters
  • chan.stopToken() to query stop state
• Adds optional timeout support to blocking operations:
  • send(..., timeout = default(Duration)) -> bool
  • recv(..., timeout = default(Duration)) -> bool
  • timeout logic only activates when timeout != default(Duration)
• Adds auto-drain on final channel destroy:
  • remaining queued items are destroyed in-place before channel memory is freed

Why this should be merged

Today, blocking channel operations can wait indefinitely unless callers build their own cancellation/time budget logic around them. This PR moves those concerns into the primitive itself, which improves correctness and reduces boilerplate in every caller.

Concretely, this improves:

• Graceful shutdown: blocked producers/consumers can be unblocked cleanly via stop().
• Bounded waiting: callers can cap blocking time using Duration.
• Resource safety on teardown: queued items are destructed during final drain, reducing leak risk for non-trivial payloads.

Behavioral details

• trySend/tryRecv stay non-blocking.
• Existing callsites that used send/recv as statements continue to compile; return values are now available for timeout/stop outcomes.
• recv()/recvIso() value-returning forms now raise "channel stopped" if they cannot receive due to stop.
• Final-destroy path drains remaining queued elements in-place (=destroy on slot storage).

Tests

Added: tests/tchannels_stop_timeout.nim

Covers:

• stop unblocks blocked recv
• stop unblocks blocked send
• recv timeout behavior
• send timeout behavior
• destroy-time draining of pending items

Also validated existing channel tests still pass:

• tests/tchannels_simple.nim
• tests/tchannels_singlebuf.nim
• tests/tchannels_cooperative.nim

Risk / compatibility

• Main API change is that blocking send/recv now return bool.
• This is source-compatible for existing statement-style usage and adds explicit failure signaling for stop/timeout paths.
• Timeout implementation uses periodic polling (sleep(1ms)) because locks condvars in this runtime do not expose timed wait primitives.

[planetis-m]

Ready I think.

[planetis-m]

I use this implementation in my app: https://github.com/planetis-m/pdfocr
so it is currently being evaluated. The API additions allowed me to simplify communication between threads. For example, under fatal or stop conditions, I no longer need to drain the channels. There’s no need to add sentinel values. The only timeouts are not used.