Add the `experimental_features.msc4428_enabled` flag. When set, `/versions`
advertises `org.matrix.msc4428` and the Rust event serializer adds
`unsigned.org.matrix.msc4428.stable_id` (the sender's user ID) to every
event served to clients, including the events embedded in `unsigned`
(`redacted_because`, bundled edits, thread summaries). The field is subject
to the client's field allowlist like any other unsigned key.
`SerializeEventConfig.__init__` gains a required `msc4428_enabled` argument
(not module API).
Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>