Add interactive outlet picker (ArcSWAT-style) + example notebook

[rehsani]

Summary

• New floodpath.interactive module: leafmap-based outlet picker. Click any pixel in the DEM patch — it auto-snaps downstream to the nearest stream cell, delineates the upstream basin, and overlays it in green. Drag the outlet marker to fine-tune.
• Returns an OutletSelection carrying the snapped outlet, basin, DEM, flow grid, and streams so the rest of the pipeline (compute_hand, routing, damage) chains off it without re-fetching.
• Adds floodpath.hydrology.snap_to_stream as the public helper underneath; surfaces outside-DEM-bbox clicks as ValueError so GUI callers handle one branch.
• Adds examples/pick_outlet.ipynb walking the full DEM → flow → streams → outlet → HAND → flood → population → damage chain at Kigali, Rwanda.
• Optional dep: pip install floodpath[interactive] (leafmap, ipyleaflet, matplotlib).

Test plan

• pytest -m "not integration" passes (343 tests, +3 snap + 8 interactive vs. baseline)
• Smoke-run end-to-end at Kigali defaults: DEM fetch, flow accumulation, stream extraction, picker construction, snap, basin delineation, HAND, flood at 5 m water level, WorldPop fetch (RWA), GHSL fetch + JRC damage curve all return sensible outputs
• Manual map test in Jupyter: click → snap → basin overlay; drag marker → re-snap; outside-domain click silently ignored