feat: add calc_lycd_land_values — Least You Can Do land valuation

[drussellmrichie]

Summary

Adds calc_lycd_land_values() to land.py, implementing the Least You Can Do (LYCD) method of land valuation described by Andelson/Tideman.

How it works

For each geographic cell the method:

1. Computes the median market value and median lot size of improved (non-vacant) parcels.
2. Derives a uniform local land rate:

   local_land_rate = (median_market_value × land_alloc) / median_lot_size
   

3. Applies that rate to every parcel in the cell:

   land_value = local_land_rate × parcel_lot_size
   

Because land value is driven by the typical parcel in an area rather than each parcel's own improvement value, the method avoids the absurd side-by-side disparities that arise from naively multiplying each parcel's market value by a fixed allocation fraction.

Key parameters

• subarea_field (str | None) — column whose values define geographic subareas (e.g. "neighborhood", "census_tract"). When supplied, one rate is derived per (subarea, model_group) cell, capturing neighbourhood-to-neighbourhood price variation that a flat county-wide rate per land-use type would miss. When None, one rate is derived per model_group (the minimal viable case).
• min_improved_per_cell (int, default 10) — cells with fewer than this many improved parcels fall back to the model-group rate, then to the global rate.
• land_alloc (float | dict | None) — fixed fraction, per-group dict, or None to auto-derive from the vacant/improved per-unit value ratio (same cell → group → global fallback chain).

Output columns added to df

Column	Description
lycd_land_alloc	Land allocation fraction used for the cell
lycd_local_land_rate	Implied per-area-unit land rate for the cell
lycd_land_value	Resulting land value (clamped to [0, market_value])

Test plan

• Smoke test: call with subarea_field=None on a small synthetic DataFrame and verify lycd_land_value equals local_land_rate × land_area
• Smoke test: call with subarea_field="neighborhood" and verify cells below min_improved_per_cell fall back to group rate
• Verify land_alloc=None auto-derives from vacant/improved ratio and falls back gracefully when a group has no vacant parcels

🤖 Generated with Claude Code

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