explorer: round-trip specs for remaining URL state surfaces (follow-up to #209)

[rdhyee]

Follow-up to #209 / PR #214. PR #214 lands CI-safe Playwright specs covering the URL-state slices that #203/#205/#210/#212 actually fixed — but #209's checklist had more on it. Tracking the deferrals:

• pid deep-link + active filters (hydration vs. sourceFilterSQL() / facet filters — current boot+hashchange handlers don't appear to apply these)
• Valid h3 deep-link round-trip — open #h3=<valid> in fresh tab; cluster card hydrates; URL persists across pan/zoom
• search + search_scope round-trip — ?search=...&search_scope=area restores input, scope toggle, re-runs query
• Facet filters: ?sources=, ?material=, ?context=, ?object_type= each round-trip
• view=table mode restore
• Search-result flight: clicking a search row flies camera; URL captures the new camera and clears stale selection

Out-of-scope here (intentionally):

• Search-result row click selection (explorer: URL state contract micro-fixes (post-#203 follow-ups) #207 item 8) needs working FTS fixture data — separate spec once that's available.
• Cold-cache live-site investigation stays in tests/playwright/url_roundtrip_investigation.js (the manual diagnostic), too slow / network-sensitive for CI.

Refs PR #214 (which closed the camera+mode+sub-threshold-pan+h3-null-clear slice).