Two ways the event rules reported handled events as unhandled (error
severity, high confidence):
1. A handler on the event base or an intermediate group. `on<SyncEvent>`
with a switch inside handles every subclass, but handlers were matched
to dispatches by exact name, so every dispatched subclass was reported
by event-without-handler and the base handler itself by dead-handler.
Variants now record the supertype they were enumerated through, and
coverage follows it: an event is handled when it or an ancestor is
registered, and a handler is live when anything at or below it is
dispatched or constructed. A group handler still does not cover a
sibling outside the group (UploadAbandoned stays reported).
2. Blocs sharing an event type. Only the first owner of an event type was
kept, so a second bloc's handlers were ignored: an event only it handles
was reported unhandled, and its dead handlers were never examined. There
is now one event union per owning bloc. A dispatch does not record which
bloc it targets, so an event any sharing bloc handles counts as handled,
and an unhandled event is reported once per shared union rather than
once per bloc. Dead handlers are still judged per bloc.
Reproduced first on base_event.dart: 4 false findings before
(SyncStarted, UploadRetryNow unhandled; SyncEvent, UploadRetry dead) and
the shared case (ShareDismissed falsely unhandled, ShareBetaBloc's dead
ShareExpired missed). Each is now a contract control.
No change on nuwayze (102 findings) or tick (2): byte-identical before
and after, since neither uses these patterns.
