site: avoid loading the full search index on every page

[TankTechnology]

Evidence

The deployed _verso-search/searchIndex.js is approximately 36 MB raw and 6.3 MB gzip, and it is referenced by ordinary reader pages.

Boundary

Preserve Verso search semantics while preventing chapter navigation from downloading and parsing the full index before the reader opens search.

Acceptance

• Chapter pages do not request the full index during initial load.
• Search returns the same representative results.
• Desktop and 390-pixel mobile browser checks have no console errors.