Specs: when the source goes out, and who owes the offer

AGPL section 13 starts at the cutover, not at the announcement: the fork is
a modified AGPL program and its users reach it over a network. Settled
today that the source is released after the cutover, with the links live
then, and in the meantime the server's users are the operator's household,
so the people owed an offer and the people holding the repository are the
same people.

Anyone migrating their own server inherits that obligation on their first
day and has no such overlap, so the migration tool says so at the end of a
successful run instead of leaving it to be discovered.

docs/spec/cutover.md

@@ -272,6 +272,20 @@ 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.
 
+## Source, and AGPL section 13
+
+The fork is a modified AGPL program, so everyone who interacts with it over
+a network has to be offered its Corresponding Source. That obligation starts
+when the fork begins serving, which is the cutover, not the announcement.
+
+Settled (John, 2026-09-19): the source is released after the cutover and the
+links go live then. In between, the only people the server serves are the
+operator's household, so the set of users owed an offer and the set of
+people holding the repository are the same set. A migration by anyone else
+inherits the obligation on their own first day, which is worth saying in the
+migration tool's output rather than leaving them to find it
+(`migration.md`).
+
 ## The first week
 
 - **Certificate renewal.** The one thing that fails quietly and late. Watch

docs/spec/migration.md

@@ -192,6 +192,14 @@ the verification, reports the timings, and removes the copy. It answers "how
 long will this take, and will it work" without touching the running server,
 and it is what the operator should run first.
 
+## What the operator inherits
+
+INBUXA is AGPL, and a migrated server is one its users reach over a network,
+so from its first day its operator owes those users an offer of the
+Corresponding Source (AGPL section 13). The tool should say so once, at the
+end of a successful migration, rather than leaving an obligation to be
+discovered. It is satisfied by a link, and INBUXA's own is published.
+
 ## Open
 
 - Where it lives: a command in ihasmail-oneshot, a sibling to