A proposition that a prosperous farmer brought to the California ballot would threaten two ambitious water and rail projects that Gov. Jerry Brown is pushing, requiring voters’ OK before launching any state building project requiring $2 billion or more in revenue bonds. Proposition 53, if voters approve it on Nov. 8, could force Brown’s administration to hold statewide votes on funding for two controversial mega-projects that he is trying to get well underway before he leaves office in 2018.