Add a Maximum Economic Yield (MEY) section to the MSY fishery lecture

Follow-up to #763 (the new MSY lecture, lectures/msy_fishery.md).

The lecture stops at MSY and notes in its summary that MSY "ignores economic costs and prices." MEY is the natural next step and fits the intro level well — it's the same one-variable concave maximization, reusing the existing Gordon–Schaefer setup and parameters.

Proposal: add a short Maximum Economic Yield section (after Sustainable yield, before Randomness) covering:

• sustainable rent $\pi(e) = p,y^*(e) - c,e$, maximized at $e_{MEY}$
• the three reference points on one diagram: $e_{MEY} < e_{MSY} < e_{\text{open access}}$, where the open-access (zero-rent) point delivers Gordon's tragedy-of-the-commons result — currently gordon1954 is cited but this result isn't covered
• one line tying it back to the collapse analysis: since $x_{MEY} > x_{MSY} = K/2$, MEY also leaves a larger safety buffer (more profit and more robust)
• brief real-world case: Australia's Northern Prawn Fishery switching its target from MSY to MEY

Scope: keep it static (maximise sustainable rent). Defer dynamic/discounted MEY (Clark's modified golden rule) to the advanced series.

Refs: add grafton2007 (Grafton, Kompas & Hilborn, "Economics of Overexploitation Revisited," Science); clark1990 already in the bib.

Start by reading lectures/msy_fishery.md, especially the Sustainable yield and Randomness sections, and inspect its existing Gordon–Schaefer setup and diagrams. Add the scoped static MEY section, the three-reference-point diagram, the Northern Prawn Fishery example, and the grafton2007 bibliography entry; done means the lecture and references build cleanly without introducing dynamic MEY material.