131 reject single continuation branch with payload

// Test: a SINGLE continuation branch carrying a payload is illegal.
//
// A single output must be expressed as a bare return `-> T`, never as a
// one-branch tagged union. This is the with-payload sibling of the existing
// single-void-branch rejection (parser.zig: "single branch with no payload is
// redundant"). One branch is never the right shape: zero outputs => void event,
// one output => `-> T`, two-or-more => a real tagged union with >= 2 branches.
//
// WRONG: one branch, with a payload — should be `~event run { x: i64 } -> i64`.
~event run { x: i64 }
| total i64