docs(changelog): move the entry into a changelog.d fragment

Main now takes pending entries as changelog.d/<pr>-<slug>.md
files (#787), so the entry leaves CHANGELOG.md for changelog.d/686-terrain-heights-bounds.md.

changelog.d/686-terrain-heights-bounds.md

@@ -0,0 +1,9 @@
+- Bound what a caller can grow through `/api/terrain/heights`. Every distinct
+  5-decimal point was a permanent in-memory entry, a permanent line in an
+  uncapped `.gev-cache/terrain-heights.json`, and a live upstream call, with
+  nothing limiting the rate. The point cache now holds at most 50,000 entries
+  and evicts oldest-first, a cache file above
+  24 MB is not read back, and the route is limited to 90 requests/minute per
+  client (270 global). Evicting a point costs one refetch, never a wrong
+  answer, since terrain does not move. Ordinary use is unaffected: a full
+  `test:track` run drives 10 requests, peaking at 8/min (daikaginza, #686).