[ak2, ak_aiyagari, two_computation] Add figure captions and lw=2 per the style guide #1080

Follow-up from the style pass on #1078 (now live in publish-2026oct09). Two figure conventions from the manual's figures page are only partly applied in these three lectures. Neither affects correctness, so this is a cleanup task.

1. Every code-generated figure should carry a caption

The manual asks for mystnb: figure: caption metadata on every code-generated figure, rendered as "Fig N. caption". #1078 added captions to the single-panel figures whose embedded titles were removed, but the multi-panel figures are still uncaptioned.

These counts cover the main text only and skip figures drawn by helper functions such as plot_surface_grid in two_computation, so the real number is a little higher.

Constraints to respect:

• Captions go on the {code-cell} directive, not inside the plotting code, and the figure needs a name for numref.
• Captions are lowercase after the first letter and about five or six words.
• Figures inside exercise and solution directives must stay uncaptioned, because the caption wraps the image in a LaTeX float and breaks the PDF build. The new exercises in all three lectures print tables rather than figures, so this mostly does not bite, but keep it in mind.
• Multi-panel figures may keep their per-axis set_title calls; the caption describes the figure as a whole.

2. Use lw=2 for line charts

Rule 8 on the same page asks for lw=2 on all matplotlib line charts. two_computation already does this for its experiment figures. ak2 and ak_aiyagari have no line widths set anywhere, including inside the plot() methods of ClosedFormTrans and AK2, and path_iteration's verbose plots.

Suggested approach

One PR per lecture keeps the diffs reviewable, since the captions touch many cells. The plot() methods in ak2 produce one figure each, so a caption on the cell that calls them is enough.