Any merger in the drug and biotech industries has one sure effect: Biogen Idec Inc. shares will jump. Sure enough, Pfizer Inc. agreed yesterday to buy Wyeth, and Biogen stock climbed $1.09, to $48.51. Biogen is routinely mentioned in speculation about drug industry mergers, though there's no evidence of any activity. The Cambridge company fought off the advances of activist investor Carl Icahn in a proxy fight last year.