Debug TOC Demo Navigation

Goal

Fix http://localhost:3002/blocks/toc-demo so TOC clicks navigate to the target heading and the target flash is visible.

Phases

• Reproduce on /blocks/toc-demo and confirm the failure shape.
• Check existing learnings and relevant route/component patterns.
• Isolate whether the bug lives in the blocks demo wrapper, generated registry payload, or TOC runtime.
• Implement the minimal durable fix.
• Verify with tests, lint, registry rebuild if needed, and browser proof on /blocks/toc-demo.

Notes

• Browser repro already shows all TOC rows incorrectly render aria-current=true on /blocks/toc-demo.
• Clicking a TOC row on that route does not scroll and does not set nav highlight.
• Avoid broad www build unless absolutely needed.