Keep browser back navigation anchored after hash jumps

[xyjk0511]

PR draft: Fix Safari back button scroll restoration after reference jumps

Closes #72

Summary

This adds a small, dependency-free scroll restoration helper to header.html.

When a reader clicks an in-page reference link, the script stores the current scroll position for the current page. If the browser later navigates back via history or restores the page from the back-forward cache, it restores that saved position after layout has had a chance to settle.

Why

Safari can lose the expected scroll position after jumping from a deep section to an appendix/reference anchor and then pressing Back. The page also has delayed layout work from rendered math/content, so a single immediate scrollTo is not always enough. The helper retries briefly and then stops.

Scope

• Does not change book content.
• Does not change styles.
• Does not add dependencies.
• Only runs for same-page hash links and browser history/page-cache restoration.

Verification

• Extracted the injected JavaScript from header.html and ran node --check successfully.
• Reviewed the diff to confirm the change is limited to header.html.

Not tested

• Manual Safari reproduction, because Safari is not available in this Windows environment.