Fix parameter order in solve_model function calls in ifp_advanced

The solve_model function signature expects (ifp, c_init, a_init) and returns (c_out, a_out),
but all call sites were passing parameters in the wrong order (a_init, c_init) and
expecting returns in the wrong order (a_out, c_out).

This fixes all 5 call sites in the lecture to use the correct parameter and return order:
- Line 490: Fixed initial solve_model call
- Line 497: Fixed timed solve_model call
- Line 645: Fixed simulation section call
- Line 737: Fixed return volatility loop call
- Line 814: Fixed income volatility loop call

Fixes #759

Author: mmcky

lectures/ifp_advanced.md

@@ -487,15 +487,15 @@ a_init = σ_init.copy()
 Let's generate an approximation solution with JAX:
 
 ```{code-cell} ipython3
-a_star, σ_star = solve_model(ifp, a_init, σ_init)
+σ_star, a_star = solve_model(ifp, σ_init, a_init)
 ```
 
 Let's try it again with a timer.
 
 ```{code-cell} python3
 with qe.Timer(precision=8):
-    a_star, σ_star = solve_model(ifp, a_init, σ_init)
-    a_star.block_until_ready()
+    σ_star, a_star = solve_model(ifp, σ_init, a_init)
+    σ_star.block_until_ready()
 ```
 
 ## Simulation
@@ -642,7 +642,7 @@ s_grid = ifp.s_grid
 n_z = len(ifp.P)
 a_init = s_grid[:, None] * jnp.ones(n_z)
 c_init = a_init
-a_vec, c_vec = solve_model(ifp, a_init, c_init)
+c_vec, a_vec = solve_model(ifp, c_init, a_init)
 assets = compute_asset_stationary(c_vec, a_vec, ifp, num_households=200_000)
 
 # Compute Gini coefficient for the plot
@@ -734,8 +734,8 @@ for a_r in a_r_vals:
     n_z_temp = len(ifp_temp.P)
     a_init_temp = s_grid_temp[:, None] * jnp.ones(n_z_temp)
     c_init_temp = a_init_temp
-    a_vec_temp, c_vec_temp = solve_model(
-        ifp_temp, a_init_temp, c_init_temp
+    c_vec_temp, a_vec_temp = solve_model(
+        ifp_temp, c_init_temp, a_init_temp
     )
 
     # Simulate households
@@ -811,8 +811,8 @@ for a_y in a_y_vals:
     n_z_temp = len(ifp_temp.P)
     a_init_temp = s_grid_temp[:, None] * jnp.ones(n_z_temp)
     c_init_temp = a_init_temp
-    a_vec_temp, c_vec_temp = solve_model(
-        ifp_temp, a_init_temp, c_init_temp
+    c_vec_temp, a_vec_temp = solve_model(
+        ifp_temp, c_init_temp, a_init_temp
     )
 
     # Simulate households