Initial commit: Four-Quadrant Balance Sheet Matrix (FQBM) framework

Co-authored-by: Cursor <cursoragent@cursor.com>

tests/test_differential_model.py

@@ -0,0 +1,30 @@
+"""Tests for Part XIV: differential model and stability checks."""
+
+import numpy as np
+from fqbm.state import FQBMState
+from fqbm.system.differential_model import (
+    solve_trajectory,
+    check_stability,
+    DifferentialParams,
+)
+
+
+def test_solve_trajectory():
+    x0 = FQBMState(B=100, R=50, Loans=500, Deposits=600, E_b=50, S=1.0)
+    params = DifferentialParams(monetary_shock=1.0, credit_cycle=0.01)
+    t, X = solve_trajectory(x0, (0, 1), params, t_eval=np.linspace(0, 1, 11))
+    assert t.shape[0] == 11
+    assert X.shape == (11, 12)
+    assert X[-1, 0] > X[0, 0]  # B increased
+    assert X[-1, 1] > X[0, 1]  # R increased
+    assert X[-1, 3] >= X[0, 3]  # Loans
+
+
+def test_check_stability():
+    x = FQBMState(R=100, Loans=1000, E_b=100)
+    checks = check_stability(x, k=0.08, reserve_threshold=50, debt_gdp=0.5, r=0.05, g=0.02, primary_balance_gdp=0.02)
+    assert "CR_ok" in checks
+    assert "reserves_ok" in checks
+    assert "debt_sustainable" in checks
+    assert checks["CR_ok"] is True
+    assert checks["reserves_ok"] is True