[ak_aiyagari] Fix inconsistent household reference (hh → household)

[sudayuga]

Summary

Several functions incorrectly referenced a global variable hh instead of using the household argument passed to them.
This caused silent inconsistency and potential runtime failure if hh is not defined in the global scope.

Fix

• Replaced all hh.* references with household.*
• Ensured dimensional consistency (J, num_state)
• Updated calls to backwards_opt, popu_dist, compute_aggregates,
  and solve_backwards to use the injected household object

[jstac]

many thanks @sudayuga !

@HumphreyYang , could you please review?

[HumphreyYang]

Many thanks @sudayuga, this looks perfect to me!

Hi @mmcky, I think this is ready to merge. I will do some minor cleanups on other parts of the lecture once it is merged!