Issue #3613235: A visitor with no live interactive step sees the neutral page instead of the chain sticky messages

When a run advances onto automatic or operator steps (an off-site payment whose settlement carried the run through confirmation, or an operator validation), an instance-scoped visitor returning to their page has no live interactive step and gets the neutral "nothing to do right now" page, even though the run chain still carries a sticky milestone (a booking-confirmed after payment, an awaiting-confirmation while an operator validates).

Fix: in CapabilityResolver::resolve(), when an instance-scoped grant finds no live interactive step, surface the current chain messages (its sticky milestones) instead of the neutral page, falling back to neutral only when the chain carries no message.

Covered by a new ChainTest case: a run parked on a non-interactive step after a sticky milestone shows that milestone to the instance-scoped visitor. Full orchestra_interaction kernel suite stays green (70 tests).

Ref #3613235.