PresentationController class

The write face of a presentation: the half only the hosting core holds.

One object, two types — exactly as Completer/Future does. Core creates, emits and settles here; the plugin gets presentation and can only read, subscribe and cancel. Without the split a plugin could settle its own presentation, and the exactly-once guarantee would be a convention rather than a fact.

The guarantees are enforced here, not by whoever calls in, so a fake host in a test inherits them for free:

• G1 settle takes effect exactly once; later calls are no-ops.
• G3 markDisplaying emits DisplayedSignal at most once, and never after settlement.
• G4 no signal is delivered after settlement. A ClickedSignal between releaseHold and settle is delivered — that window is the non-blocking case.
• G5 CampaignPresentation.cancel is idempotent and safe in any state.
• G8 CampaignPresentation.holdReleased settles exactly once and never after the outcome: settle releases the hold first if nothing already did, so a plugin can bind CEP unblock to it unconditionally.