region-filter: cross-entry Back, E2E API-count flake, history.state pin (follow-up to #549) #556

Follow-ups from the #549 re-review (Minimax, APPROVE with nits):

1. Cross-entry Back. Going back to an older history entry that still carries a link ?region= reapplies and saves that region over a choice the user made later on another entry. For example, #/nodes?region=SJC → #/channels → pick OAK → Back gives SJC. Same-entry is fixed. Possible fix: mark an entry whose link has already been applied in history.state, as Analytics does for follow-ups from #203, #204 and #206 reviews (P3: test gaps, inactive-card wording, history defaults, Hash Stats audit) #208, and skip the override on Back/Forward.
2. E2E flake risk. test-issue-531-region-url-e2e.js section 2 compares API counts that depend on whether Packets' HopResolver is already ready (1 of 10 runs failed under load). Leave /api/nodes and /api/iata-coords out of the Packets comparison, or wait for HopResolver.ready().
3. Test gap. syncUrlRegion keeps history.state (replaceState(window.history.state, …)), but no test pins it; the replaceState(null, …) mutant survives.

Relates to #549 / #531.