Clarify utility function notation to avoid confusion with unemployment compensation

Changed utility function parameter from 'c' to 'x' to distinguish between:
- The consumption argument of the utility function u(x)
- The unemployment compensation parameter c

This prevents confusion when we write expressions like u(c), where c is
the specific value of unemployment compensation being passed to the
utility function.

Updated in:
- Job Search II: def u(c, γ) → def u(x, γ)
- Job Search III: def u(c, γ) → def u(x, γ)
- Job Search IV: def u(c, γ) → def u(x, γ)
- Job Search V: u(c) = ln(c) → u(x) = ln(x)

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <[email protected]>

Author: jstac

lectures/mccall_fitted_vfi.md

@@ -233,8 +233,8 @@ $$
 We use the same CRRA utility function as in the discrete case:
 
 ```{code-cell} ipython3
-def u(c, γ):
-    return (c**(1 - γ) - 1) / (1 - γ)
+def u(x, γ):
+    return (x**(1 - γ) - 1) / (1 - γ)
 ```
 
 Here's our model structure using a NamedTuple.

lectures/mccall_model_with_sep_markov.md

@@ -208,8 +208,8 @@ The optimal policy turns out to be a reservation wage strategy: accept all wages
 The default utility function is a CRRA utility function
 
 ```{code-cell} ipython3
-def u(c, γ):
-    return (c**(1 - γ) - 1) / (1 - γ)
+def u(x, γ):
+    return (x**(1 - γ) - 1) / (1 - γ)
 ```
 
 Let's set up a `Model` class to store information needed to solve the model.

lectures/mccall_model_with_separation.md

@@ -239,8 +239,8 @@ Let's now implement a solution method based on the two Bellman equations
 The default utility function is a CRRA utility function
 
 ```{code-cell} ipython3
-def u(c, γ):
-    return (c**(1 - γ) - 1) / (1 - γ)
+def u(x, γ):
+    return (x**(1 - γ) - 1) / (1 - γ)
 ```
 
 Also, here's a default wage distribution, based around the BetaBinomial

lectures/mccall_persist_trans.md

@@ -146,7 +146,7 @@ $$
 \frac{u(w)}{1-\beta} \geq f^*(z)
 $$
 
-For utility, we take $u(c) = \ln(c)$.
+For utility, we take $u(x) = \ln(x)$.
 
 The reservation wage is the wage where equality holds in the last expression.