Conditional logic in UI steps (IF/ELSE/BRANCH)

[Explorer-64]

Problem

Real UIs have state. The current step vocabulary is a flat linear sequence
with no branching. Examples of flows the spec cannot currently express:

• A "Complete" button that shows "Undo" if already logged today
• Optional fields that change the flow depending on user input
• A form that shows different fields based on a previous selection

An agent following linear steps mechanically will fail on any flow
that branches.

Proposed addition

A conditional syntax for v2.1.0:

IF selector_exists [data-agent-id="undo-button"]
  CLICK [data-agent-id="undo-button"]
ELSE
  CLICK [data-agent-id="complete-button"]
END

Source

Raised during implementation against an app with stateful UI elements
that change based on prior user actions.