Add changelog fragment for the Data Library

changelog.d/1604-data-library.md

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+- Add the Data Library. New regional and niche sources no longer join the
+  Data Layers panel: **Browse data library** at its foot lists live entries
+  (a manifest each in `src/library/entries/`, fetched from the provider, never
+  bundled) with their licence, coverage and what this instance needs to serve
+  them, and **Add** puts one under a Library heading. Datasets (GeoJSON, CSV,
+  ArcGIS), raster tiles and host-layer packs are supported; selections persist
+  and travel in share links as `lib=`, separate from the layer tokens; one
+  generic `/api/library` proxy serves entries without CORS. Seeded with NASA
+  EONET events, EIA and WRI power plants, NWS alerts, NOAA tide stations,
+  Smithsonian volcanoes, NASA Black Marble, OpenSeaMap seamarks, and the
+  opt-in IMF PortWatch and CCTV packs. FEMA USA Structures and the
+  National Bridge Inventory load what is in view once you zoom in, and
+  Addresscloud's EPC ratings stream from Source Cooperative's GeoParquet on
+  S3, coloured A–G with live legend counts. Open tile overlays (geology,
+  biodiversity, population, surface water, national aerial photos and maps)
+  join GDACS alerts and EPA Superfund and TRI facilities. 64 aerial photo
+  layers from the Open Imagery Register (national programmes across Europe,
+  NSW, NAIP and US state, county and city services, WMS and ArcGIS export
+  included) show their resolution, true-ortho flag, years flown and refresh
+  cycle in Browse; aerial photos stay above every other drape, are clipped to
+  their coverage, and the providers' blank fill no longer whitens the globe.
+  Kartverket's open Norwegian maps (topographic, nautical charts, hillshade,
+  terrain relief, sea depths, boundaries and the cadastre) and Mapterhorn's
+  global terrain, drawn in the browser as shaded relief with an elevation
+  legend, are in the library too. Stat tiles head the dialog. A library layer's row has
+  Zoom to layer and Remove icons, and the dialog has Zoom and a confirmed
+  Remove all (asturksever, #1604).