Webform interaction: going back leaves the run stuck on the form, and back/cancel shows a confirmation screen

Problem

In the Webform interaction flow, when a visitor goes back from a later step to the form step (a loop, for example a payment step routing a "back" outcome to the form), resubmitting the reopened form leaves the run stuck on the form node: the dispatcher keeps redirecting to the form and the step never advances.

Separately, the back and cancel actions show a generic "Are you sure? This action cannot be undone." confirmation page, which is the wrong experience for plain navigation.

Steps to reproduce

1. Build a workflow with a webform wait step that loops: the form step goes to a later wait step, and the later step routes a "back" outcome to the form step.
2. Submit the form, reach the later step, then go back.
3. The reopened form is shown; submit it again.

Result: the run sits on the form step instead of advancing. The back and cancel links also show a confirmation page first.

Root cause

• The resume handler resumed the token id named by the submission, but a loop re-enters the node with a fresh token, so that id is already consumed.
• The orchestra_interaction element relied on a plugin default property for prepopulation. Webform tracks prepopulation from the stored #prepopulate property, not the plugin default, so the element never captured the capability token from the dispatcher link.
• The back and cancel outcomes were resumed through a confirmation form, even though the signed capability token in the link is already the gate.

Proposed resolution

• Resume by node: add InteractionResolver::parkedTokenAtNode() and have OrchestraResumeHandler resume the token parked on that node now, so a re-submit after going back advances instead of looping on the form.
• Force prepopulation on the orchestra_interaction element in initialize(), so it is genuinely zero-config and always captures the token.
• Resume visitor outcomes in one click: InteractionController::signal() verifies the token, checks the outcome is one the parked step allows, and resumes immediately. Remove the confirmation form.

Adds a functional test driving the full dispatcher to webform to resume to back to resubmit path over HTTP, plus kernel coverage of the loop re-entry and the one-click signal action.