Background work follow-up: unpolled progress events, Run(action=tests), and a measured side-by-side #6961

Follow-up from #6944. The founder closed it for 0.10.2 on 2026-10-12. Its slices that needed no decision shipped in #6957:

• wait reports progress;
• the cancel receipt names a person's kill;
• the 30 s background move says so plainly.

Three items remain, each a product call:

(a) Progress without polling. Should the engine push mid-run progress or stall events to the model when it hasn't polled? That would be a new user-role runtime event, logged in the session, and it could nudge the model mid-turn. The #6944 addendum says completion "prompts the model only if the model asked to be told".

(b) Run(action=tests). It runs raw cargo test with a 30-minute kill and never moves to the background. Options:

• delete it in favour of bash. This changes the tool catalog and the session-pinned cached prefix for new sessions. Note that scripts/dev-test.sh exists only in this repo.
• or route it through bash's backgrounding path.

(c) Measured side-by-side. A wall-clock comparison of a long cargo run before and after the background changes, on a real build.

Done when

• (a) and (b) are decided and either built or written down as won't-do.
• (c) has a recorded measurement.

Refs #6944