Playground demo: an agent-harness transcript rendered from real protocol event payloads -- a completed message, collapsible reasoning, and a fault.

Exercises Raxol.UI.Components.Harness.{MessageBlock, ReasoningBlock, ErrorBlock} directly. Only ReasoningBlock carries interactive state (its own expanded flag), toggled through its real handle_event/3 -- mirrors how ScrollAnchorDemo forwards raw key events into Viewport.