Improve beforeload event

In #276 (CB-14188) the `beforeload` event was added.
This event currently uses the same callback mechanism as other events, but it is different in one important aspect: instead of just notifying that something happened, it expects an action to be called. It would be good to allow a synchronous callback instead.

Review issue #276 (CB-14188) and the existing beforeload event callback path; this issue names no implementation files or tests. Identify the relevant platform entry points, then confirm that beforeload supports a synchronous callback and that its behavior is covered on the supported platforms.

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