[bartlomieju]

Summary

• uv_write in the uv compat layer was firing write callbacks synchronously when try_write succeeded fully or failed. This caused a RefCell double-borrow panic ("RefCell already borrowed") when the caller (e.g. StreamWrap.write_buffer) held an OpState borrow during the uv_write call, and the synchronous callback re-entered JS which called another op borrowing OpState.
• Changed both TCP and TTY uv_write implementations to always queue completed writes with a pre-determined status, deferring callbacks to poll_tcp_handle/poll_tty_handle in the event loop. This matches real libuv behavior where uv_write never fires callbacks synchronously.
• Added a status field to WritePending to support pre-completed entries that skip I/O in the drain loop.

Closes #32891

Test plan

• Added regression test uv_write_callback_is_deferred verifying callback does not fire synchronously from uv_write
• All 58 uv_compat tests pass
• Node.js unit tests pass (buffer, net, http, http2, tty)

🤖 Generated with Claude Code

[littledivy]

LGTM