Aggregate downscaled scenario outputs to county and state level

[divine7022]

Summary

Implements county- and state-level aggregation for management scenario outputs, completing the downscaling workflow for Phase 3 scenario analysis. Includes visualisation updates .

Closes #168

Changes

041_aggregate_to_county.R

• Aggregates field-level predictions to county level by scenario, PFT, and carbon pool
• Generates state-level summaries (California totals) per scenario
• Processes delta predictions (start->end carbon change) with scenario grouping
• Outputs: county_aggregated_preds.csv, county_aggregated_deltas.csv, state_summaries.csv, aggregation_metadata.json

043_county_level_plots.R

• County stock maps now include scenario in filename and subtitle
• Removed county-level density choropleth maps (per CARB: difficult to interpret at county scale)
• Added field-level point maps showing carbon density per field (more informative spatial granularity)
• Delta maps updated with scenario awareness

Documentation:

• Updated docs/workflow_documentation.md with new output file descriptions
• Updated docs/CHANGELOG.md

Dependencies

• Requires outputs from #167 (downscaled_preds.csv, downscaled_deltas.csv)