docs(changelog): add the terrain test cache isolation fragment

Co-Authored-By: Claude Opus 5.5 <[email protected]>
Claude-Session: https://claude.ai/code/session_01BNTtHAa65nobzJ7jp2Ltr4

changelog.d/1244-terrain-test-cache-isolation.md

@@ -0,0 +1,4 @@
+- The terrain-heights bounds tests no longer read the checkout's
+  `.gev-cache/terrain-heights.json`. Each test gets a cache file of its own, so
+  `npm test` no longer fails after the dev server has cached terrain near
+  Austin (KnottyDyes, #1244).