Calculate budgets

Calculate budget for water in a domain and compute timeseries of inflow and outflow local boundaries as well as surface fluxes to understand if differences between models in i.e. precipitation are caused by internal domain processes or forced through the larger-scale boundaries.

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### Describe alternatives you've considered

No files, tests, or entry points are identified. First map the existing Python model-evaluation and diagnostics paths, then define the water-budget inputs, boundary and surface-flux outputs, and comparison criteria; done means reproducible timeseries that distinguish internal from boundary-forced differences.

Written by the indexing model from the issue text.