docs: preflight leaves the roadmap and the design contract

Owner, 2026-10-18: "Preflight там почему остался?" It was not dropped with a
condition for return: what it was for shipped in other forms - its checks on
Server › Health (health-setup-checks, 2.11.0), its test e-mail as a form of
its own (test-email, 2.12.0) - and its mockup went in 2c23f4c. The entry
under Dropped, the route row and the Who-sees-what mention go; the history
is in git and the CHANGELOG. The port-check entry still says it came from
the former preflight, which is where it came from.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docs/roadmap.md

@@ -134,10 +134,6 @@ without that it is not offered again.
   line "last connection from outside on each port", read from `mail.log`, and
   a command in the help to run from one's own machine. **Returns if:** the
   owner accepts an outside probe, or asks for one of the two substitutes.
-- **preflight as a page of its own** (dropped 2026-10-13): its installation
-  checks went to Server › Health (health-setup-checks, `2.11.0`) and its test
-  e-mail to a form of its own (test-email, `2.12.0`). Its plan,
-  `plans/preflight.md`, was removed and stays in git history.
 - **A domain administrator's rows in the queues** (decided 2026-10-18:
   "Пока только Global. Пересмотр по необходимости."). Since `2.16.0` both
   queues are tables split by the recipient's domain, so the rows of a