fix(infrastructure): ground-sample only stem bases the camera sees

scene.sampleHeight reads only surface rendered for the current view. For a
data centre or dam base outside the frustum or over the horizon it read
through the Earth (about -6,344 km over Ashburn on Esri), failed the 9,000 m
check and retried every 2 s indefinitely, one offscreen render per record.
Over Ashburn that held rest frames at 58-589 ms. Unseen bases are now
skipped without being stamped, so each is sampled on the first walk that
sees it, and they no longer arm the retry render.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

docs/CURRENT-STATE.md

@@ -1959,7 +1959,9 @@ during continuous motion. Disable/re-enable resets selection cleanly.
 Close-up stem sizing uses camera-to-feature distance without a 5 km minimum.
 Unchanged overlay entries do not request another frame; moved positions,
 changed membership and re-enabling still publish. Ground sampling waits for
-visible globe tiles to settle and uses valid loaded terrain as a floor while
+visible globe tiles to settle, samples only markers whose base is in the
+camera's view and above the horizon (an unseen base reads through the Earth
+and would retry every 2 s), and uses valid loaded terrain as a floor while
 retaining rooftop and below-sea-level elevations. A hidden globe does not
 constrain photoreal geometry, and retry work remains bounded. Already sampled
 nearby markers follow higher settled terrain on existing bounded walks, without