Implement TODOs in revised first half of MSY lecture

- Add a figure showing the shape of the logistic growth curve G(x);
  move the G definition above it.
- Refactor plot_45 to take the one-year map as a function argument, so
  the no-fishing 45-degree diagram works before fishing/effort and the
  update() function are introduced; update both call sites to pass
  lambdas.
- Add the two-effort 45-degree diagram (staircases omitted) showing
  x*(e) falling with effort.
- Fix the multi-effort growth-vs-harvest figure: use explicit efforts
  [12.5, 25, 37.5] and G(K/2) for the y-limit (e_msy/MSY are not yet
  defined there), and fix the label typo.
- Restore the grid-search computation of e_msy, MSY, x_msy in the MSY
  section.
- Drop the under/over-fishing framing from the yield-effort curve per
  review note, since under randomness no constant policy is fully safe.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: jstac