feat(hydrobasins): gate snap to Pfaf-12 ranges and prove full-depth integration

[CooperBigFoot]

Summary

• reject HydroRIVERS builds before source loading when the selected Pfaf range excludes level 12
• filter snap stems exclusively against merged Pfaf-12 units and emit the corresponding HFX reference level
• add red-first range, merged-region, deterministic-order, and canonical full-depth Greek integration coverage

Testing

• uv run python -m unittest test_build_adapter.SnapLevelGatingTests test_build_adapter.HydroRiversSnapTests.test_two_region_multi_level_snap_filters_only_pfaf_12_ids_in_deterministic_order
• uv sync
• uv run python -m unittest discover -p "test_*.py" (125 tests passed; 4 HFX_BIN-dependent tests skipped; canonical Greek full-depth integration ran and passed)