Specs: a draft runbook for the cutover

Steps 1 to 3 of SPEC §7 are met, so the remaining one is running the fork
as the mail server. The draft covers the sequence on the host, what to
check before letting mail flow, and what to watch in the first week.

Two things it refuses to gloss: rolling back stops being a snapshot restore
the moment the fork accepts a message, because nobody has tested whether
the Enterprise build reads a store the fork has written; and certificate
renewal is the failure that arrives 90 days late and quietly, on the one
path the suites couldn't settle.

Nothing in it has been rehearsed. The rehearsal on a copy is step 2 of
"Before the day", and it is what turns "the data opens" into "the server
runs on it".

docs/spec/SPEC.md

@@ -544,7 +544,7 @@ Steps 1 to 3 are met as of 2026-09-19: the fork builds and its suites pass
 (`container-tests.md`), and all eight compat tests pass against a copy of
 INBUXA's store, with the caveat that two of them have nothing to read and
 the tenant administrators' own view is not yet recorded
-(`compat-tests.md`). What remains is step 4.
+(`compat-tests.md`). What remains is step 4, drafted as `cutover.md`.
 5. Only then: public repository and announcement, with the INBUXA fork of
    ihasmail released alongside. Public ihasmail stays Stalwart-facing (§5).