Phase 2d: hydro discovers gauges via the USGS IV bBox query across the
coverage box (thresholds stay NWPS-cached; unrated sites never emit) instead
of a hand-listed site set; satpass seeds a single centroid observer derived
from the box. Completes wiring all native env adapters to the universal
coverage bbox. Coverage governs when set; each adapter's config is fallback.
Co-authored-by: Matt Johnson <mj@k7zvx.com>
Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
