Delay stable promotion for new .0 releases on the latest track

## Problem

`promote-snap-train` (justfile) promotes every track from `edge` straight to `stable` on a daily cron, with no age check. For the latest track, if the version being promoted is a new `.0` patch release (i.e. the first release of a new upstream minor/major version), this can push an untested release to `stable` the same day it lands in `edge`.

## Proposal

For the **latest track** only: if the revision in `edge` is a `.0` release, only promote it to `stable` once it has been sitting in `edge` for at least a week (or matches whatever soak period upstream uses for that release, if longer).

Non-`.0` patch releases and older tracks keep the current same-day `edge` → `stable` behavior.

## Why

Gives `.0` releases, which carry the highest regression risk, a minimum soak time in `edge` before reaching users on `stable`.

Start with the `promote-snap-train` recipe in the justfile and trace how it identifies the latest track, `.0` releases, and the time spent in `edge`. Implement the minimum soak period for qualifying latest-track releases while preserving same-day promotion for non-`.0` releases and older tracks, then verify the promotion behavior.

Written by the indexing model from the issue text.