[jampukka]

Follow-up to #243. PyramidBounds reads the written tile pyramid (gpkg_tile_matrix_set plus the srs_id -> CRS mapping) and works out the WGS84 bbox and center. PMTilesConvert uses it instead of hardcoded Finland defaults. GeneratorControl and TileSources do the startup validation. MvtTilesExtension in hakunapi-gpkg writes the gpkg_extensions row.

Behaviour changes

• PMTilesConvert without --bbox or --center derives both from the pyramid. The Finland defaults are gone. For a CRS that is not 4326 or 3857 and has no projection factory on the classpath, it asks for --bbox.
• A .collections key that clashes with a .collections key (source id equal to range name) is rejected at startup with a clear message.
• A range filter that uses a CQL2 function fails at startup with "Unmapped Function (the generator registers no functions)". The README documents this.
• The generated GeoPackage gets a gpkg_extensions row for the vector tile table, so other readers don't treat it as raster. GPKG now creates gpkg_extensions with IF NOT EXISTS and also creates it for tile-only files.
• The README has a worked example and a PMTilesConvert section.