Revise MSY lecture: rename effort to e, yield to y*(e), add early graphic

- Rename effort symbol E -> e and sustainable yield Y(E) -> y*(e)
  throughout prose, math, and code, matching the x*(e) notation.
- Introduce y*(e) graphically with a growth-vs-harvest figure right
  where it is defined, marking x*(e) and y*(e) at effort e=20.
- Shrink the two 45-degree diagrams by 10% (figsize 5.5 -> 4.95).

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

Author: jstac