Specs: point a stock ihasmail at the migrated server afterwards

Asked for by John, 2026-09-19. Public ihasmail is Stalwart-facing and knows
nothing about INBUXA, so running an unmodified one against the migrated
server checks something the fork's own suites cannot: that a client written
for upstream still works.

Each difference it finds is one of two things, and the point is to say
which: a regression against upstream's contract, which the fork's tests
would not catch because they test the fork; or a feature that now expects
INBUXA's own front ends, which belongs in the contract and the release
notes rather than in a user's surprise.

After mail is flowing, not as a gate. It informs the contract; it doesn't
block a cutover.

docs/spec/cutover.md

@@ -4,7 +4,9 @@ Status: draft, 2026-09-19. Nothing here has been rehearsed yet.
 
 SPEC.md §7 step 4. This run is also the first run of the migration tool
 INBUXA will ship (`migration.md`): what is done by hand here is what that
-tool automates, including its rollback. Steps 1 to 3 are met: the fork builds and its suites pass
+tool automates, including its rollback.
+
+Steps 1 to 3 are met: the fork builds and its suites pass
 (`container-tests.md`), and all eight compat tests pass against a copy of
 INBUXA's store (`compat-tests.md`). What is left is running it as the mail
 server.
@@ -143,6 +145,29 @@ never been checked, and checking it is worth an hour beforehand rather than
 an argument at 2am. Until it is checked, treat the fork's store as
 forward-only.
 
+## After: stock ihasmail against the fork
+
+Once the migration is done, point an **unmodified public ihasmail**
+container at the new server — it can run on a laptop, since it talks JMAP
+and OAuth over the network (SPEC.md §5.3) — and work through what it can
+do. It needs an OAuth client it is allowed to use and a reachable URL for
+the server, nothing else.
+
+This is the one check that comes from outside the fork's own world. Public
+ihasmail is Stalwart-facing by design (SPEC.md §5) and knows nothing about
+INBUXA, so whatever it can no longer do is one of two things, and both are
+worth knowing:
+
+- a **regression**: the fork broke something upstream's clients rely on,
+  which the fork's own suites wouldn't catch because they test the fork;
+- a **gap by design**: a feature that now needs INBUXA's own front ends,
+  which belongs in the contract (SPEC.md §5.2) and in the release notes
+  rather than being discovered by a user.
+
+Write down which of the two each difference is. Do it after mail is
+flowing, not as a gate: it informs the contract, it doesn't block the
+cutover.
+
 ## The first week
 
 - **Certificate renewal.** The one thing that fails quietly and late. Watch