Make library search and category filters deep-linkable

[HMAKT99]

Summary

The homepage library's filter state (search query, active category, page) lived only in memory. As a result:

• a filtered or searched view couldn't be shared or bookmarked,
• a reload dropped the filters back to the default view, and
• the browser back/forward buttons didn't restore previous filter states.

This wires that state into the URL query string so a slice of the library is directly linkable — a natural fit for a catalog that's explicitly built for SEO/GEO and agent discovery (llms.txt, catalog.json, etc.).

Behavior

• Search → ?q=…, category → ?category=engineering, page → ?page=2. Defaults (empty query, all, page 1) are omitted so the canonical homepage URL stays clean.
• On load and on popstate, state is restored from the URL. The category is validated against the available filter chips (unknown values fall back to all) and the page is parsed/clamped by the existing pagination logic, so hand-edited or stale URLs degrade gracefully.
• Uses history.replaceState (guarded for environments without it), so typing in the search box doesn't flood the history stack.

Implementation notes

• Extracted the category-activation logic into a reusable applyCategory() (used by both the click handler and URL restore).
• syncUrlState() is called at the end of updateLibrary(), which already runs on every search/category/pagination change, so there's a single write path. It only calls replaceState when the URL actually changes.
• No-ops cleanly on pages without the library (the searchInput/results guards already short-circuit updateLibrary).

Verification

• node scripts/check.mjs → Loop Library checks passed.
• node --check site/script.js
• Builders (build-skill-catalog, build-loop-pages, build-social-images) → no artifact drift.
• Manual: searching, switching categories, and paging update the URL; reloading and back/forward restore the same view; ?category=bogus / ?page=99 normalize to a valid state.

[HMAKT99]

This one's a feature rather than a bug fix, so completely understand if it's not a direction you want — feel free to close it, no hard feelings.

The idea: the homepage filter state (search, category, page) lives only in memory, so a filtered view can't be shared or bookmarked, a reload drops it, and back/forward don't restore it. This reflects it in the URL (?q=, ?category=, ?page=), omitting defaults to keep canonical URLs clean, and restores + validates it on load and popstate. It felt like a natural fit for a catalog that's explicitly built for linking and agent discovery, but I get that it adds surface area.

If you're open to it but want it scoped differently (e.g. query-only, no page param), I'm glad to trim. Check suite + builders pass with no artifact drift.

[mberman84]

The URL state is bookmarkable, but three blocking issues remain:

1. syncUrlState() starts with a fresh URLSearchParams, so loading ?utm_source=newsletter&q=agent immediately deletes utm_source and any other URL state the library does not own. Initialize from window.location.search, then set/delete only q, category, and page.
2. Every interaction uses replaceState. Browser verification confirmed that after changing category or page, Back leaves the library instead of restoring the prior filter state, so the new popstate handler cannot deliver the behavior claimed by the PR. Use pushState for meaningful category/pagination navigation boundaries; reserve replaceState for initial canonicalization or transient search typing.
3. This changes site/script.js without bumping the shared versioned script URL. Update the emitted/validated asset version and regenerate affected pages.

[HMAKT99]

All three addressed:

1. syncUrlState() now seeds from window.location.search and only sets/deletes q/category/page, so utm_source and any other foreign params survive.
2. Category and pagination changes use pushState (Back now restores the prior filter state); replaceState is reserved for initial canonicalization and transient search typing. popstate stays read-only.
3. Bumped script.js to 20260621-url-state everywhere + regenerated the loop pages.
   Check suite + builders green.

[HMAKT99]

@mberman84 ready for re-review whenever you have time — foreign params preserved, pushState on navigation boundaries, version bumped. Appreciate the detailed feedback.