Why: SR-B10 (RDM requirements/01 V2.1, ruling J-03) defines the task-level
dual-speed pattern: a fast lane gives immediate feedback while deep work runs
in background with queryable progress and results posted back to the inbox.
The base system had the background spawn/notice machinery but nothing the
user could see while it ran or collect afterwards, so heavy tasks just made
the user wait on a synchronous turn.
Evidence: go test ./... -count=1 green (32 pkgs); new runmgr tests cover the
card lifecycle on success (progress card -> done/ok with result summary and
artifact line, card.created/updated/status events) and on failure
(done/danger with the error), plus the cards==nil no-op path.
Impact: spawnBackground now registers an inbox progress card linked to the
child run and rewrites it with the result on completion; the delegate_task
description and base-method working prompt teach the fast-feedback-first
pattern; a #background mock demo exercises the whole path end to end.
Refs: RDM docs/requirements/01 UR-27/SR-B10; .tmp 03-V2 §任务快慢双速通用模式;
Dev task feature/dual-speed-tasks.
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