explorer: URL state contract micro-fixes (post-#203 follow-ups)

[rdhyee]

Cluster of small, targeted URL-state contract fixes surfaced by Codex's retrospective review of #203 + #205. Each item is independently shippable; bundling for tracking, not for one PR.

Items

• Low-altitude-without-mode=point round-trip (Codex item 4). The early-write fix in explorer: fix URL state round-trip (closes #201 parts A+B) #203 can produce URLs like alt=8000 without mode=point if the user copies during a pending point-mode transition. Boot only force-enters point mode when the URL says mode=point, so the round-trip lands in cluster mode at low altitude — not what the user saw. Fix shape: boot path should enter point mode based on altitude (alt < ENTER_POINT_ALT) AND/OR the URL flag, not the URL flag alone. Update tryEnterPointModeIfNeeded() call at boot end to fire whenever altitude is below threshold, regardless of ih.mode.

• h3 hashchange null-result asymmetry (Codex item 6). When fetchClusterByH3(state.h3) returns null in the hashchange handler (explorer.qmd:~2568-2580), the UI is cleared but viewer._globeState.selectedH3 is NOT cleared. Boot does clear it (line ~2590). The asymmetry means an invalid/filtered h3 can be re-emitted later by buildHash(). Fix shape: clear viewer._globeState.selectedH3 in the hashchange null branch to match the boot behavior.

• Selection state with search-result flights (Codex item 8). Search-result clicks fly the camera but don't set or clear selectedPid / selectedH3. The URL then becomes "new camera, old selection". Fix shape: search-result click handler should either set selection to the searched-for sample or clear stale selection before flying.

• Avoid pushState during boot/hash hydration (Codex item 3). tryEnterPointModeIfNeeded() at boot eventually calls enterPointMode() with default pushHistory, so boot hydration can add a browser history entry. Boot and hashchange should reconcile state without growing history. Fix shape: thread an explicit { pushHistory: false } option through the boot call site.

Why bundled

Each is a small, localized fix in explorer.qmd. None unblocks the others. Codex flagged them as a group; tracking them together keeps the URL state contract's edges visible while we work through them. Individual PRs are fine.

Refs

• explorer: 'Samples in View' shows the 5000 fetch budget, not real count + URL round-trip doesn't reproduce view #201 / explorer: fix URL state round-trip (closes #201 parts A+B) #203 / explorer: add camera.moveEnd URL-write backstop (closes #204) #205 — original URL round-trip work
• PR explorer: fix URL state round-trip (closes #201 parts A+B) #203's retrospective Codex review comment — full context
• Explorer state contract: URL/DOM/widget-state inventory + search-as-global-filter decision #164 — Explorer state contract (these items belong in that inventory once stabilized)