Add an "Open" link to reach a parked step's interaction from the instance trace

Problem/Motivation

The instance view's process trace lists each token of a run with an operations dropbutton (the signal actions and the fire-timer action). When a step parks on an external interaction (a payment page, a message, a redirect), an operator has no way to reach that interaction page from the trace: to preview it, or to copy its link and resend it, they must wait for the notification that normally delivers it.

Proposed resolution

Add a generic alter hook, hook_orchestra_ui_token_operations_alter(array &$links, TokenInterface $token), that lets any module add its own per-token operations to the trace. orchestra_interaction implements it: for a parked token whose node declares an interaction, it prepends an "Open" link to the visitor's step page, carrying the per-step capability token. The link is offered only on the admin-gated instance view.

To build that link as a render element, InteractionUrlsInterface gains stepLink(), returning a Url object for the same target as stepUrlForToken() (which now delegates to it); the capability and assignment gateways implement it too.

Remaining tasks

• Review.