NARR reader hardening + TEMP gap visibility (CREST_AI 08a219d)

update_temp_narr: retry NARR download on HTTP 429/5xx with backoff

Health-check fix batch (CREST_AI 3dca271)

sync: episode final product takes peak observed flow from the scorer's full USGS series (manifest hydro understated 15.7 vs 34.5 on 03274650); run_nowcast_all.py parity

V22: virtual points — 2,676 ungauged HydroBASINS lev07 outlets fill CONUS gauge gaps; hindcast via upstream-gauge injection (speed scheme forced) + nearest-gauge parameter regionalization

V16: NARR temperature source + DEM-based temp extrapolation — NARR air.2m auto-appended to temp/ tars (generic member names, reader fallback); [TEMPForcing] DEM= grid aggregated to the temp grid enables EF5's -6.5 C/km lapse whenever SNOW17 is on