Fix remaining k references to use s for savings

Changed remaining instances where k was used instead of s:
- Mathematical notation: x = k + σ(k) → x = s + σ(s)
- Added missing inline comment in K_crra: x_i = s_i + c_i

This completes the transition to using 's' for savings throughout,
maintaining consistency with the exogenous savings grid terminology.

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

Co-Authored-By: Claude <noreply@anthropic.com>

Author: jstac

lectures/cake_eating_egm_jax.md

@@ -311,7 +311,7 @@ def K_crra(
     c_out = compute_c_vectorized(s_grid)
 
     # Determine corresponding endogenous grid
-    x_out = s_grid + c_out
+    x_out = s_grid + c_out  # x_i = s_i + c_i
 
     return c_out, x_out
 ```
@@ -390,7 +390,7 @@ plt.show()
 
 Note that the plots for $\gamma > 1$ do not cover the entire x-axis range shown.
 
-This is because the endogenous grid $x = k + \sigma(k)$ depends on the consumption policy, which varies with $\gamma$.
+This is because the endogenous grid $x = s + \sigma(s)$ depends on the consumption policy, which varies with $\gamma$.
 
 Let's check the maximum deviation between the log utility case ($\gamma = 1.0$) and values approaching from above.