Adds `SessionView { Terminal, Structured }` to maestro-protocol, a
defaulted `view` field on `Session` and `CreateSessionRequest`, and
sidecar round-trip tests for both a legacy payload with no `view` key
and a `Structured` session. Nothing reads the field yet; every other
struct literal across the workspace picks up `Default::default()`
(Terminal) to keep compiling.
Gitea #133.