Desktop GeoPackage of the habitat export: segments cut from FWA at their measures #332

If done: a desktop user gets one GeoPackage per watershed group with link's habitat for each published config: the export's rows joined to FWA lines cut at each segment's measures, plus a scenarios layer from the manifest. If never: desktop use needs fwapg and a SQL join.

The optional desktop export from #308, deferred from its first cut.

• Source: habitat.fwa_v1_segment after lnk_db_load(), or the parts directly.
• Geometry: ST_LocateBetween (or fwapg's equivalent) on whse_basemapping.fwa_stream_networks_sp by blue_line_key and the two measures. Measures do not abut exactly (sub-cm float slop, RUNBOOK §6d).
• One layer per config × species, or one long layer with config / species columns: decide with the QGIS consumer.
• scenarios layer: one row per config from the manifest's configs[] (run ids, config hash, thresholds_logged, discharge).
• Writer: sf links GDAL 3.8.5 here; GeoPackage needs nothing newer.

Relates to #308, NewGraphEnvironment/floodplains#123.