Replace `write_reqs_pending` (unsigned int counter) in
`uv_stream_connection_fields` with `write_queue` (struct uv__queue),
mirroring the Unix stream implementation. Add `struct uv__queue queue`
to `UV_WRITE_PRIVATE_FIELDS` so each `uv_write_t` can be linked.
This enables iterating over in-flight write requests individually, which
is used in `uv__tcp_try_cancel_reqs` to replace the blanket `CancelIo`
call with per-request `CancelIoEx` calls (resolving the TODO comment
that anticipated this change).
Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>
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