Feature: 2D water contamination support (lakes, seas, estuaries)

[backlundtransform]

Summary

The current WaterContaminationSimulator implements 1D advection-diffusion along river networks.
To support lake, sea, and estuary scenarios, a 2D spread engine is needed.

Motivation

RiskZ needs to model contamination in different water body types — not just rivers but also
lakes (quiescent spread), coastal waters (tidal mixing), and estuaries (bidirectional flow).

Proposed scope

1. WaterBodyType enum — River, Lake, Estuary, Sea
2. 2D advection-diffusion solver for lake/sea scenarios (grid-based Eulerian scheme)
3. Builder extensions — WithLake(), WithSea(), WithEstuary() on WaterContaminationScenarioBuilder
4. Wind-driven circulation (optional, for lakes)
5. Tidal forcing (optional, for estuaries/seas)
6. Strategy routing — select 1D river or 2D solver based on water body type

Current state

The completed roadmap (docs/completed/WaterContaminationRoadMap.md) explicitly lists
2D lateral dispersion, lake stratification, and tidal mixing as "Future Extensions (Out of Scope for MVP)".

Notes

• No assignee for now — this is a future enhancement tracked for planning purposes.
• The existing 1D river engine should remain unchanged.