Add critical stream transition behavior notes to engagement programs

[ZLeventer]

Summary

Adds three missing behavioral notes to the engagement program stream transition documentation that practitioners frequently discover through trial and error:

• Cast-time evaluation: Transition rules evaluate at cast time, not in real time. A person who qualifies mid-cast still receives the current stream's content for that cast and moves to the new stream before the next cast.
• Single-stream membership: A person can only belong to one stream at a time — they are never in multiple streams simultaneously.
• Position reset on transition: When a person transitions to a new stream, they start receiving content from the top of that stream, even if they had exhausted all content in their previous stream.

These are common gotchas that cause unexpected behavior in production engagement programs, especially around exhaustion handling and transition timing.

Test plan

• Verify callout blocks render correctly on the Experience League page
• Confirm no formatting issues with the existing content above