fix(terrain): keep known heights when a terrain request fails

A request with any point the proxy cannot answer gets 502, and the page
used to put every point of that chunk on its geoid, including the ones the
proxy had real (cached or stale) heights for. During an upstream 429 pause
that drops cached ground under every chunk that also holds a new point.

The 502 body now carries `partial`: the request's real heights in order,
null for points without one, left out when there are none. The terrain
request service attaches it to the error it throws, and the page caches
those heights and uses its geoid only for the rest. `get_terrain_height`
still reports the failure as before.

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

docs/CURRENT-STATE.md

@@ -3092,7 +3092,10 @@ down | auth-failed` (plus the unchanged `missing-key`/`unsupported`) with
 >   outlasts its retries, the proxy pauses upstream calls for the `Retry-After`
 >   (60 seconds when none, at most 10 minutes). A request whose points are all
 >   cached, fresh or stale, is still served; a request with any uncached point
->   gets 502 and the page uses its geoid fallback for that chunk. Each request
+>   gets 502, whose `partial` lists the real heights it has (null for the rest)
+>   when it has any, and the page keeps those and uses its geoid fallback only
+>   for the points without one. The same holds when the upstream answers a
+>   point with a null height. Each request
 >   that was waiting on the upstream when the 429 came back logs one warning;
 >   requests refused during the pause do not log. Client geoid
 >   fallbacks wait 60 seconds before retrying and self-heal to Re:Earth on the