docs(changelog): add the dead-code cleanup entry

changelog.d/1321-dead-code-cleanup.md

@@ -0,0 +1,6 @@
+- Remove dead code across the repository: three modules nothing imports
+  (`src/ui.js`, `src/standalone/ui.js`, `src/data/localLayers.js`), exports
+  and locals nothing reads, a duplicate copy of the bundled dams data and an
+  unused ship model. The app no longer builds an unused default wind layer
+  and Bhote Koshi locator layer when it loads. Package exports are unchanged
+  (Sameh Khamis, #1321).