docs(changelog): add the terrain partial-heights entry

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

changelog.d/1456-terrain-partial-heights.md

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+- A terrain request the proxy can only partly answer (a 429 pause, or a point
+  the upstream returns without a height) now keeps the real heights it has
+  instead of putting the whole chunk on the geoid (quanticsoul4772, #1456).