Add time-adjusted Saint-Quentin models

[RCoeurjoly]

Summary

Adds a Saint-Quentin, France test jurisdiction using metric units and public data.

This adds:

• a script to prepare cadastre and DVF data
• time-adjusted sale prices
• main and vacant model runs with naive_area, local_area, and lightgbm
• a small fixture test and README

Validation

• venv/bin/pytest tests/test_saint_quentin_fixture.py
• venv/bin/pytest
• Ran the full Saint-Quentin local workflow and saved model outputs

Notes

Raw data and generated outputs are not committed. The local run used 43,189 parcels, 7,966 sales, and 2,702 vacant sales. The vacant-land model is an early benchmark, not final valuation output.

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