Fix schedule action delay after refresh

[chaptersix]

What changed?

• Record a buffered start's desired time when refresh observes the prior action complete.
• Gate the new state mutation at scheduler workflow version 13.
• Cover the refresh path with an action-delay metric regression test.

Why?

A completion discovered by refresh previously left DesiredTime unset, causing schedule_action_delay to fall back to the scheduled time instead of the prior action's close time.

How did you test it?

• added new unit test(s)
• go test -tags test_dep ./service/worker/scheduler -count=1

Potential risks

The state change is version-gated through the existing tweakables mutable side effect so replay of version-12 histories retains the old behavior.