Place rice in a fine-mesh strainer and rinse under cold water, swishing with your hand, until water runs mostly clear - about 1 minute. This removes excess starch so the grains stay separate. Shake off excess water.

In a medium saucepan, heat oil over medium heat. Add drained rice and cook, stirring constantly, until grains look slightly translucent at the edges and smell toasty - about 2 minutes. Add water and salt, stir once to combine, and bring to a rolling boil.

Cover tightly, reduce heat to the lowest setting, and cook undisturbed for 15 minutes. Remove from heat and let stand, still covered, for 5 minutes. The rice should be tender and all liquid absorbed.

Remove lid and fluff rice gently with a fork, lifting from the bottom to separate grains. While rice is still hot, drizzle lime juice evenly over the surface, add zest, butter if using, and most of the cilantro. Fold together with the fork until well distributed.

Taste and add more salt or lime juice as needed - the rice should taste bright and lightly tangy. Transfer to a serving bowl, sprinkle with remaining cilantro, and serve immediately while warm.