docs(changelog): add the #1712 fragment

Co-Authored-By: Claude Fable 5.2 <[email protected]>

changelog.d/1712-ais-store-prune.md

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+- The AIS vessel store no longer walks every vessel for each position message.
+  Expiry runs on the existing 15 s tick, the cap evicts in update order, and a
+  vessel's static data is dropped with it (mazy1998, #1712).