fix(infrastructure): stop idle republishing and respect settled terrain

Skip unchanged overlay cohorts while detecting moved positions and lifecycle changes. Defer ground sampling until visible terrain settles, retain the loaded surface as a floor, and preserve roof heights and bounded retries. Keep regression and idle-render gates intact.

CHANGELOG.md

@@ -13,6 +13,10 @@ of current runtime behavior, see [`docs/CURRENT-STATE.md`](docs/CURRENT-STATE.md
 
 ## [Unreleased]
 
+- Unchanged local infrastructure overlays no longer sustain idle rendering.
+  Ground samples wait for visible terrain to settle and cannot place a marker
+  below its loaded surface; roofs and valid below-sea-level heights are retained.
+
 - Datacenter and dam marker stems use bounded, zoom-dependent active sets with
   stable selection during camera motion. Close-up stems scale to the actual
   camera distance; source totals and submarine cables remain unchanged.