Flood overlay water rendering + Trinidad coastal resilience demo

[brendancol]

Summary

• Flood-as-water rendering: New overlay_as_water flag in the render kernel. When a flood_* layer is active, flooded pixels (overlay > 0.5) render with the full ocean water shader (deep blue base, procedural wave normals, Fresnel reflections, specular highlights) instead of colormap blending. Cycling flood_1m → flood_2m → … flood_10m with G key creates a rising-flood animation.
• Fixed flood masks: Non-flooded pixels are now NaN (not 0.0), so dry land shows the satellite/terrain base instead of a black mask.
• Info text in help menu: New info_text parameter for explore() appends a MODEL INFO section to the paginated help pages. Supports key-less entries that span the full column width.
• Trinidad & Tobago Coastal Resilience demo: New example script with bathtub inundation model across both islands — Copernicus 30m DEM, Overture buildings/roads color-coded by surge risk, cyberpunk minimap, wind animation.

Test plan

• Run trinidad_tobago_coastal_resilience.py and verify satellite tiles are visible on dry land (no black mask)
• Cycle layers with G — flood layers should render as water with reflections/waves
• Press H to cycle help pages — MODEL INFO section should appear on last page
• Verify non-flood overlay layers (surge_risk) still render with normal colormap blending