Stage 5 Increment 3 shadow residual-capacity fallback

[agurgone]

Implements Stage 5 Increment 3 as a shadow-only residual-capacity fallback after Increment 2.

What changed:

• Added a pure residual-capacity helper for the provisional DSTI proxy and illiquid-asset fallback.
• Wired the Stage 5 shadow plan into Country._set_household_target_demand().
• Recorded new shadow diagnostics on households: dsti_headroom, dsti_maximum_loan_size, dsti_cap_binding, borrow_planned, liquidation_planned, shadow_credit_requested, forced_liquidation_amount, residual_shortfall_after_caps.
• Added a consumer-credit config hook for the calibrated proxy parameters.
• Added unit tests for the helper and the country/household wiring.

Verification:

• uv run pytest tests/test_macromodel/unit/test_agents/test_households/func/test_residual_capacity_fallback.py tests/test_macromodel/unit/test_agents/test_households/test_households.py tests/test_macromodel/unit/test_country/test_country.py -q
• uv run ruff check macromodel/agents/households/func/residual_capacity_fallback.py macromodel/agents/households/households.py macromodel/country/country.py macromodel/configurations/country_configuration.py tests/test_macromodel/unit/test_agents/test_households/func/test_residual_capacity_fallback.py tests/test_macromodel/unit/test_agents/test_households/test_households.py tests/test_macromodel/unit/test_country/test_country.py

Note: run_model/run_model.ipynb has an unrelated pre-existing local modification and was not included in the commit.