config: ship the first migration, which exercises the chain end to end

[bdchatham]

Runs the machinery end to end. The rewrites existed and nothing had run one.

This appends the first migration, raises the schema version to 2 in the same change, and records what the
step does to its own fixture, which is what freezes it.

It renames the state-commit write mode cosmos_only to memiavl_only. The two names describe the same
routing: releases before v6.6 used the first and this one uses the second, and the reader has translated
the old name on every start since. Moving the file once is what lets a node stop depending on that
translation.

The reader keeps accepting the old name, and has to. Nothing migrates app.toml, so a node still reading
its settings from there depends on it. This migration moves sei.toml and changes no reader.

Five tests cover the chain this binary actually carries rather than one built for a test, because that is
the only chain that runs on a node. A file an operator wrote is upgraded in place: the value moves, the
version moves, the release that moved it is recorded, the operator's comment survives, an unrelated setting
in the same section survives, and the step reports exactly one key changed. A file already holding the
current spelling moves version and reports nothing changed, which is most of a fleet. A file that never
wrote the key gains nothing. Running the upgrade twice leaves the file byte-identical. A preview writes
nothing.

The sixth is the one the migration exists for. Every other test stops at the file. This upgrades a file,
boots with it, and asks the reader that consumes the key what it got: a migration whose result does not
reach the reader has moved a file and nothing else.