fix(overpass): reserve singleflight before disk cache reads

docs/CURRENT-STATE.md

@@ -1,5 +1,11 @@
 # God's Eye View Current State
 
+Overpass reserves each normalized query before its disk-cache lookup. The same
+reservation covers upstream quota admission, concurrency, fetch and cache
+publication; duplicate callers join the complete transaction and share stale
+fallback on failure. Settled and rejected reservations are removed only while
+they still own their map entry.
+
 Shared distance checks use `src/geoDistance.js` with `{ lat, lon }` points and
 the IUGG mean spherical Earth radius, 6371.0088 km. `greatCircleKm` is
 canonical; `greatCircleMeters` is its exact metre conversion. Missing,