fix(terrain): key the 429 pause on status, never shorten it, and document it

Review follow-ups: the upstream error now carries its HTTP status and the
pause checks error.status instead of the message text; concurrent 429s
extend the pause with Math.max rather than a shorter Retry-After cutting a
longer one short; the per-chunk re-check says it is for multi-chunk
requests; the first 429 test runs its backoff on mocked timers.
CURRENT-STATE describes the pause.

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

src/data/terrainHeightsProxy.js

@@ -145,6 +145,7 @@ export async function fetchTerrainChunkWithRetry(
       const res = await fetchImpl(url, signal ? { signal } : {});
       if (!res.ok) {
         const error = new Error(`HTTP ${res.status}`);
+        error.status = res.status;
         error.retryable = res.status === 429 || res.status >= 500;
         error.retryAfter = res.headers?.get?.('retry-after') ?? null;
         throw error;