Add a 'reset earlier messages' option to render-only message stacking

Render-only messages accumulate in a chain: the page shows the latest message plus any sticky ones. There is no way to show a message on its own, clearing the sticky milestones kept before it, so a terminal message (a cancellation, a rejection) appears next to the happy-path milestones that preceded it.

Add a reset option, the inverse of sticky: when the chain reaches a reset message, every earlier sticky milestone is dropped and the page starts fresh from there.

The two chain-behavior flags are grouped under a stacking group on the message settings, so the config is self-documenting and the options read clearly as a render-only concern: stacking.keep (previously the flat sticky) and stacking.reset. RenderOnlyInteractionInterface gains resetsEarlier() next to isSticky(), MessageInteraction gains the grouped checkboxes, and the chain aggregation in InteractionController drops the sticky trail when it reaches a reset message. Pre-1.0, so the config shape change ships without an update hook; the example workflows are migrated in the same change.