fix: pass generation through with action to discard stale ones (#260)

Previously, we discarded `ActionAccepted` only based on state kind, and
the union of states in which to discard actions was missing a case for
`Running`. When the browser navigated back or forward and hit an
exception, that triggered an eager state read, which then raced with the
FrameNavigated(BackForwardCacheRestore) event, ending up Running +
ActionAccepted.

Bumping generations and threading the generation through the action
selection in the runner (in BrowserState) solves this in a robust way.