Add Python state target preparation pipeline

[donboyd5]

Implement a python pipeline that derives per-state target files from IRS SOI and other geographic shares, applied to TMD national totals.

Pipeline:

• prepare_targets.py: CLI entry point
  python -m tmd.areas.prepare_targets --scope states
• prepare/soi_state_data.py: read and process raw SOI state CSVs
• prepare/target_sharing.py: compute TMD × SOI shares with OA rescaling
• prepare/target_file_writer.py: expand JSON recipe into per-state CSVs
• prepare/extended_targets.py: Census SALT, SOI credit, and additional variable targets using external geographic distribution data
• prepare/constants.py: AGI bins, variable mappings, state metadata
• prepare/census_population.py: embedded Census state population data

Directory restructure:

• SOI state data moved to prepare/data/soi_states/
• Recipes moved to prepare/recipes/
• Old prepare_states/ infrastructure removed

Key design choices:

• SOI shares rescaled so 51 states sum to 1.0 (excludes "Other Areas")
• Filing-status count targets excluded from $1M+ AGI bin (dual variable analysis showed these are the dominant source of weight distortion)
• Extended targets use Census S&L finance data for SALT distribution and SOI credit data for EITC/CTC
• CD support deferred to a future PR

Target files are not committed — they are fast to regenerate (~4 seconds).

[martinholmer]

@donboyd5, How would you suggest reviewing PR #465? It seems incomplete: that is, no use of the targets.

[donboyd5]

@martinholmer said:

@donboyd5, How would you suggest reviewing PR #465? It seems incomplete: that is, no use of the targets.

Would it work to fetch both #465 and #466 and then run the full pipeline?

make clean
make data
python -m tmd.areas.prepare_targets --scope states
python -m tmd.areas.solve_weights --scope states --workers 8  # adjust workers as appropriate
python -m tmd.areas.quality_report

I am away for the next hour but after that could provide more info if you have questions. I'm sorry, the documentation isn't quite up to snuff yet.