fix: Idempotent start

[plajdo]

Added check whether any subscriptions for current Reactor are active. This makes start function idempotent for callers, and effectively prevents creating duplicate subscriptions.

Condition: transform function must be free of side effects, as empty transform will be called multiple times, until first subscription is created for the Reactor.

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Disabled auto-start of AnyReactor type-erased type.
Previously this auto-start behaviour caused incorrectly stored Combine/non-combine subscriptions to external events from viewModels. Subscriptions were being stored under the wrapper type, which caused confusion and non-clear event routing.

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Effects:

Call to viewModel.start() now has to be explicit and will always be forwarded to correct concrete Reactor type.