explorer: expand Playwright URL round-trip coverage beyond camera+mode

[rdhyee]

Codex retrospective on #203 + #205 noted the URL round-trip harness (tests/playwright/url_roundtrip_investigation.js) only covers camera + mode. The URL state contract is larger.

Untested cases

From Codex's review:

• pid deep-link + active filters: boot/hashchange PID hydration does not appear to apply sourceFilterSQL() or facet filters, so ?sources=...#...&pid=... can hydrate a sample that the active filters exclude. Need a round-trip test.
• h3 deep-link round-trip: open #h3=... in a fresh tab; cluster card hydrates; URL persists across pan/zoom.
• search + search_scope: ?search=pottery&search_scope=area should restore the search input, scope toggle, and re-run.
• sources, material, context, object_type facet filters: each should round-trip via ?sources=, etc.
• view=table: table mode should restore.
• Search-result flight: clicking a search result flies the camera; URL captures the new camera and either reflects or clears prior selection. Confirm round-trip.
• Sub-threshold pan settles via moveEnd (regression for explorer: add camera.moveEnd URL-write backstop (closes #204) #205): a small pan with no zoom should update the URL within ~1s of mouseup.
• Cold-cache point-mode deep-link: with ?cache=disabled or a wiped browser cache, the deep-link should still round-trip — measure time-to-stable, assert eventual consistency.

Harness vs spec

The existing url_roundtrip_investigation.js is a Node script useful for manual investigation but doesn't run in CI. Plan:

1. Keep the investigation harness as a manual diagnostic (it does cold-cache live-site work that's too slow / network-sensitive for CI).
2. Extract CI-safe Playwright specs into tests/playwright/url_roundtrip.spec.js covering the items above, hitting localhost:5860 like the other specs.
3. Each spec asserts: open URL in fresh context, wait for settle, snapshot relevant state, compare against an expectation built from the source URL.

Acceptance

• New tests/playwright/url_roundtrip.spec.js (or split into focused spec files) running in CI.
• All items above pass against localhost:5860 (Quarto preview of current main).
• Either the existing investigation harness is updated to share helpers with the specs, or it's left as-is with a comment pointing to the specs.

Refs

• PR explorer: fix URL state round-trip (closes #201 parts A+B) #203 — the original URL round-trip fix
• PR explorer: add camera.moveEnd URL-write backstop (closes #204) #205 — the moveEnd backstop
• tests/playwright/url_roundtrip_investigation.js — current harness (post-explorer: fix URL state round-trip (closes #201 parts A+B) #203/explorer: add camera.moveEnd URL-write backstop (closes #204) #205)
• Explorer state contract: URL/DOM/widget-state inventory + search-as-global-filter decision #164 — Explorer state contract