The gate deadline has no stated start, so nothing can report a release past it #264

What is unclear

Chapter 55 says a release fails "when a member does not pass its analysis within the gate deadline, when the barrier does not open within it", and chapter 20 derives the deadline as the largest progressDeadlineSeconds of the members, itself startupBudget × 3.

Nothing says when that clock starts. Counted from the moment the render is applied, a release whose migration takes its ten minutes and whose analysis runs its iterations outlives a deadline sized for a pod's startup, and would be reported as held while it is going well. Counted from a member's first question, an unchanged member never starts it. Flagger enforces progressDeadlineSeconds on the Deployment itself, and waits at confirm-promotion for as long as the gate says no.

The Release Gate therefore reports no release as held by deadline (JorisJonkers-dev/delivery#3): a wrong clock would raise an urgent alert on a healthy release.

What to decide

• What the deadline measures: the barrier alone (from the first member that waits at it), each member's start, or the whole release.
• Who enforces it, and what the enforcement does: the gate answering no for good, which leaves the new copies running, or something that makes Flagger take them away.

Acceptance criteria

• Chapter 55 states when the deadline starts and what happens when it passes.
• The gate reports a release past it as held, with a reason of its own.

Part of #195.