Change `pvCanFollow` relation so positional arguments have clear roles

We should consider changing `pvCanFollow` to infix so mistakes like this are less likely.

_Originally posted by @williamdemeo in https://github.com/IntersectMBO/formal-ledger-specifications/pull/1221#discussion_r3477892714_

Start by locating the definition and usages of pvCanFollow, then read the linked discussion in PR #1221 to understand the positional-argument mistake. Determine the affected specifications and usages, and consider the change complete when the relation uses clear roles consistently without introducing ambiguity.

Written by the indexing model from the issue text.