Fix private rent target and add social rent calibration target

[vahid-ahmadi]

Summary

• Correct private rent target to use UK-wide figures
• Add new social rent calibration target

Fixes #317

Problem

The model produces inflated renter populations and social rent levels (see #317):

• Avg social rent: £8,141/yr in model vs £6,136/yr official (EHS 2023-24)
• No social rent calibration target existed to constrain this

The private rent target also used England-only renter counts (4.7m) instead of UK-wide (5.4m).

Changes

Private rent target — corrected

• Renter count: 4.7m (England-only) → 5.4m (UK-wide: EHS + devolved stats)
• Avg rent: £1,400/mo → £1,374/mo (ONS PRHI UK average, Feb 2026)
• Total: £79bn → ~£89bn

Social rent target — new

• £118/week mean social rent (EHS 2023-24) × 52 weeks × 5.0m UK social renters
• Total: ~£30.7bn
• Filters to RENT_FROM_COUNCIL or RENT_FROM_HA tenure types

Files changed

• targets/sources/housing.py: corrected private rent, added social rent target
• targets/compute/other.py: handle housing/rent_social by filtering to social tenure
• targets/build_loss_matrix.py: add housing/rent_social to dispatch
• tests/test_housing_targets.py: unit tests for all housing targets
• changelog.d/fixed/317.md

Data sources

• ONS Private Rent and House Prices UK, Feb 2026 — UK avg private rent £1,374/mo
• English Housing Survey 2023-24 — mean social rent £118/wk, 4.0m social renters in England
• EHS + devolved stats: ~5.4m private renters, ~5.0m social renters UK-wide

Test plan

• All housing targets load correctly (3 targets)
• Target values in plausible ranges
• Social rent < private rent
• All targets have 2025 values
• Targets appear in global registry
• ruff format passes