Scenario specification schema and validation for CARB HSP scenarios

[divine7022]

Adds the ability for CARB staff to define management scenarios as YAML configuration files. This is the front-end specification layer that sits between policy decisions and model inputs.

A scenario file lets you say things like "Fresno County will have 5000 acres of compost by 2030, growing to 15000 by 2045" - and the system handles the rest.

• scenario_schema.yaml - Formal definition of what goes in a scenario file. Includes all 31 HSP practices from the 2026 Practice Guidelines.
• validate_scenario.R - Checks that scenario files are valid before we try to use them. Catches mistakes like using the wrong practice for a crop type.
• scenario_to_boolean.R - Converts county-level targets into field-level assignments (which specific fields get which practices)
• scenario_to_events.R - Generates the events.json files that SIPNET actually consumes

Examples and docs (examples/scenario_bau.yaml, examples/scenario_hsp_high.yaml and docs/scenario_specification.md)

• Two example scenarios (BAU baseline and high-adoption alternative)
• User documentation explaining how to write scenario files

Details

Practices covered: All 31 from HSP 2026 Guidelines including the new ones (biochar, delta rice). Complete list with eligibility rules and NRCS codes.

Validation checks:

• Required fields present
• County names spelled correctly (all 58 CA counties)
• Practices eligible for the specified crop type
• Incompatible practices not combined (e.g., no-till + reduced-till)
• Trajectory values non-negative and sensible

county: alias: Added support for using county: Fresno instead of area: Fresno when working with the california_counties domain. Minor convenience that matches how people naturally think about it.

TODO

• Climate scenarios (GCM × SSP) - those come from Cal-Adapt, separate data pipeline
• Transition matrices for field-level dynamics - that's #160
• Integration with the actual downscaling runs - needs field data

Implements: ccmmf/organization#171