A cancel RPC could stall forever in complete-frame transport
shielding before the peer received it, bypassing the outer ack
timeout and blocking graceful supervision.
- thread one absolute deadline from `Portal.cancel_actor()` through
the private `Start` publication path
- force-close a partial-frame stream before releasing its send lock
- keep ordinary sends unbounded and preserve pending cancellation
- document the current `Start -> StartAck -> CancelAck` exchange and
link the dedicated `Cancel` msg follow-up in #506
- cover partial publication and the shared send/ack timeout budget
Prompt-IO: ai/prompt-io/opencode/20260821T023537Z_ae6f2ac3_prompt_io.md
(this patch was generated in some part by `opencode` using
`gpt-5.6-sol` (`openai`))
Gud Boi