feat: support hash section navigation

[Jalendar10]

Summary

• Treat section ids as URL hash destinations in the scroll engine
• Make route/nav clicks update the hash without changing ordinary scroll behavior
• Intercept in-engine links like #finale so template CTAs can jump to the matching section
• Document the hashNavigation option in the standalone template

Validation

• node --check skills/scroll-world/references/scrub-engine.js
• git diff --check