Streaming

The same kind of .md file, revealed a few characters at a time and parsed again on every frame, the way Markdown arrives from a model.

Comark closes the open ```mermaid fence, so a half-written diagram is still a pele node, with half its source. The component keeps one element mounted and hands each new source to Pele’s update(), so a flowchart grows a node at a time. When a frame cuts a line in half and Pele can’t draw it, the last diagram that worked stays on screen.