docs: Workflows demo SVG: pinned section headers, no real peer names

[ruvnet]

Fixes the README demo SVG merged in #3847.

• Privacy: the Conversation scene listed the recording machine's real federation peers (internal hostnames). They are replaced by neutral same-length names (alpha1, delta-hub-0, echo-worker-node-1, ...). No other file in the tree mentioned them. The earlier blob remains in git history; the names are internal hostnames, not credentials or addresses.
• Pinned section headers: the animation now keeps the current section's real header row pinned under the caption (sticky behaviour: the section at the top of the dock, or the story's section when its header is on screen).
• Same 952x964 frame, pure CSS animation, no scripts or external references, 1.27 MB. Rendered and read in headless Chrome at ~30 timestamps (boot through the title card).

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