Align OSWM basemaps with OpenFreeMap Positron and Dark Matter

[kauevestena]

This updates the Webmap basemap contract and raster-generation provenance.

• Keep the self-hosted raster PMTiles as the first choices, but make them explicitly Positron-based and Dark Matter-based.
• Add live OpenFreeMap Dark Matter and Positron as the final two basemap selector options.
• Merge live OpenFreeMap vector styles with the OSWM overlay sources/layers so thematic PMTiles, hover, popups, legends, charts, and snapshots remain part of the map.
• Bump the raster renderer contract to v3 so existing hand-styled v2 archives are regenerated.
• Record the upstream style basis in basemap generation reports and metadata.
• Add regression tests for selector ordering, live-style integration, and raster palette provenance.

Fleet baseline before this change: Curitiba/reference, Hiroshima, Lima, Milan, and Oslo all pin the same current core main SHA (e66ae89), so there is no pre-existing submodule drift to account for.