Port of upstream grok-build 0.2.103. session/prompt runs an async intake
preamble before enqueueing SessionCommand::Prompt on the actor mailbox; a
concurrent cancel could enqueue Cancel first, land on an idle actor as a
no-op, and the prompt then started a turn nothing would ever cancel.
The per-session prompt-intake lock becomes the dispatch lock: prompt()
holds it across intake as before, and cancel() now holds it around its
Cancel send, so a cancel orders behind the prompt it targets. Regression
test ported verbatim (unix-only harness, like its siblings).
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01KSy9S31pZNWXtRZNVJ1sDf
ZacharyZhang-NYandClaude Fable 5
6a4bff0d45