v0.5.2: terrain polish — icons that fall like shadows

A polish pass on 3D terrain, driven by real-world testing over the Wasatch on a Salt Lake City feed.

Fixed

• Ground icons now drape over terrain like shadows. The flat ground-projection icons were being sliced by slopes (or, in an intermediate attempt, hovering above them). Each icon is now a segmented mesh whose vertices conform to the terrain under its heading-rotated footprint — it falls across ridgelines the way an actual shadow would, and hides honestly behind mountains. Which is, after all, what a ground projection is.
• The emergency-squawk ring rides the terrain instead of pulsing at sea level (previously up to thousands of feet underground on high-elevation feeds).
• No more sea-level flash on load. Aircraft ground anchors rendered at sea level until elevation tiles finished decoding, then popped up one by one as data ticks arrived. An arriving elevation tile now re-anchors every aircraft on the next frame.

Full details in CHANGELOG.md.