Part of the Internet’s beauty is the space it affords people to take an idea and run with it . . . and run with it and run with it, until they end up miles away from where they started. Among a subset of online superfans of the BBC show “Sherlock,” solving mysteries is mere window dressing for the real story: one about unrequited love between Sherlock Holmes (Benedict Cumberbatch) and Dr. John Watson (Martin Freeman). This sort of wholesale invention, by viewers, of a romance between fictional characters who are not romantically linked on-screen is a form of something known as “shipping” (short for “relationship-ing,” the term can also refer to rooting for actual fictional couples). It is by no means limited to “Sherlock” — any form of pop culture, from “Scandal” to One Direction, is fair game — but that show has inspired vast and vivid fictional worlds, completely imagined by shippers who share screenshots, drawings and even entire books speculating about what a dalliance between Sherlock and Watson (or Johnlock, as it’s known) might look like. These fans wring meaning out of every lingering glance and anguished expression that crosses Cumberbatch’s impressively dimensioned face and superimpose their own dialogue atop these moments, amassing a trove of erotic imaginings that is in some respects more compelling than the canon, at least in the unpredictability of the plot twists. For example, one offshoot of Johnlock, known as Fawnlock, imagines Cumberbatch as an ethereal deer, complete with graceful antlers and a speckled coat — and of course his lover, Watson, cradled in his forelimbs.