Refine IFP lecture: Improve clarity, notation, and code structure

Major improvements:

- **Documentation enhancements**: Renamed to "The Household Problem", introduced
  "income fluctuation problem" terminology, added comprehensive note explaining
  timing convention and how it differs from traditional formulation

- **Mathematical precision**: Changed "measurability" to "adaptedness", used :=
  for definitions, improved EGM algorithm description with explicit definitions

- **Code refactoring**: Renamed `savings_grid` → `s` for brevity, simplified
  vmap by eliminating lambda wrapper using `in_axes=(None, 0)`, improved variable
  naming (e.g., `mu` for marginal utility)

- **Structural improvements**: Split utility setup into separate cell, added
  "Dynamics" subsection, reorganized JAX verification, improved exercise specs

- **Terminology**: Changed "Coleman-Reffett operator" → "Euler equation operator"
  in docstrings for accuracy

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

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

Author: jstac