explorer: Playwright coverage for tryEnterPointModeIfNeeded() invariants

[rdhyee]

Follow-up from #191 (issue #190 fix 2).

PR #191 introduced an idempotent tryEnterPointModeIfNeeded() helper that centralizes the cluster→point-mode transition and is called from four loadRes call sites to close a supersession liveness gap. None of the new behavior is exercised by Playwright today.

The cold-cache 60-90s repro is hard to automate (it depends on real network behavior of DuckDB-WASM 1.24.0), but the deterministic parts of the helper can be covered cheaply by mocking loadRes or stubbing viewer.camera.positionCartographic.height.

Suggested coverage

1. Short-circuit invariants — tryEnterPointModeIfNeeded() is a no-op when:

   • mode === 'point' (already there)
   • viewer.camera.positionCartographic.height >= ENTER_POINT_ALT (cluster altitude)
   • currentRes !== 8 && loading === true (another load is in flight; that path's call site is responsible for chasing)

2. Camera-handler boot path — deep-link-style URL load, mock loadRes(8, ...) to resolve true after a tick, assert enterPointMode() was called.

3. Source-filter supersession recovery (the bug round 3 closed) — set up cold-cache state with a pending res8 load, fire a source-filter change, assert that after both loads settle the user ends up in point mode, not stranded in cluster mode at point altitude.

4. Post-await re-checks — after the helper's loadRes await, assert enterPointMode() is not called when:

   • The user zoomed back out during the wait (hNow >= ENTER_POINT_ALT)
   • Another path entered point mode first (mode === 'point')
   • The load was superseded or failed (res8Ready === false)

Implementation notes

The current tests/playwright/cesium-queries.spec.js is the only Playwright file in tests/playwright/. New spec(s) probably belong alongside it. The helper lives inside the zoomWatcher OJS cell closure, so direct unit-testing is awkward — the cleanest seam is probably an integration test that drives the real explorer with a mocked db.query resolver, then asserts on mode / currentRes / phase-message text.

Acceptance

• At least one Playwright spec covering items 1, 3, and 4 above.
• Item 2 is nice-to-have but redundant with the test plan checklist in explorer: surface 'Fetching sample index…' during cold-cache boot→point-mode wait (#190 fix 2) #191 (manual verification covers it for now).
• Tests pass on the same quarto preview setup the existing tests use.

Refs

• explorer: 60–90 s 'no dots' window on cold-cache deep-link to point mode (DuckDB-WASM 1.24.0 falls back to full HTTP read) #190 — parent issue
• explorer: surface 'Fetching sample index…' during cold-cache boot→point-mode wait (#190 fix 2) #191 — PR introducing tryEnterPointModeIfNeeded()
• PR #191 round-3 review comment — original surfacing of the supersession race