Standardize value function notation in Job Search II lecture (#686)

* Standardize value function notation in McCall model with separation

This commit standardizes the value function notation in the Job Search II lecture to match the notation used in Job Search III (mccall_model_with_sep_markov.md).

Key changes:
- Renamed v(w) → v_e(w) for employed worker's value function
- Renamed h(w) → v_u(w) for unemployed worker's value function
- Updated continuation value h → v_u^* in explanations
- Updated all Bellman equations and mathematical expressions
- Updated code variables (v → v_e, h → v_u_star)
- Updated plot labels to reflect new notation
- Updated function documentation and comments

This standardization improves consistency across the job search lecture series and makes the notation more intuitive (e for employed, u for unemployed).

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

* Simplify McCall model solution to single scalar equation

Simplified the solution method by solving for v_e(w) in closed form as a
function of d, then substituting into the equation for d. This reduces the
problem from iterating on both a vector (v_e) and scalar (d) to iterating
on just the scalar d.

Key improvements:
- Added derivation showing v_e(w) = (u(w) + βαd) / (1 - β(1-α))
- Reduced to single scalar fixed-point equation for d
- Simplified code: now only iterate on scalar d instead of vector + scalar
- Compute v_e from d using closed-form expression after convergence
- More efficient: fewer variables to track during iteration
- Clearer mathematical structure

The implementation now directly reflects the simplified mathematical approach,
making it easier to understand and computationally more efficient.

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

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

* Fix Model class comment and indentation

- Changed "jitted class" to "model class" (Model is a NamedTuple, not jitted)
- Fixed indentation from 2 spaces to standard 4 spaces

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

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

* Remove unnecessary @jax.jit decorators from intermediate functions

Only kept @jax.jit on top-level user-facing functions (solve_model and
compute_reservation_wage). Removed it from intermediate functions (u,
compute_v_e, update_d) since they are automatically compiled when called
from jitted functions.

Benefits:
- Cleaner code with less decorator clutter
- Avoids redundant compilation overhead
- Follows JAX best practices of jitting at the top level

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

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

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Co-authored-by: Claude <[email protected]>

Author: jstac