Issue #3606144: Signal action lets the operator choose the branch

The instance Signal action resumed a parked token with no outcome, so a wait node's conditional outgoing flows did not match and the token fell through to the default arc (often straight to completion).

This replaces the one-click signal with a form (SignalTokenForm) that lists the parked node's branches, read from its outgoing flows' equality comparison conditions, and signals with the chosen flow's variable set to its value so that flow is taken. An "Advance with no outcome" choice keeps today's behaviour for an unconditional default arc. orchestra_ui only; uses the existing engine signal(). Covered by a kernel test.

Closes #3606144