138 reject empty ctor braces

// ============================================================================
// Test 210_138: MUST_FAIL — empty constructor braces `=> done {}` are a relic
//
// A payloadless branch constructs with its name alone: `~foo => done`.
// The braced spelling `=> done {}` used to parse to a DIFFERENT tree than
// the bare form (the braces flipped BranchConstructor.has_expressions), so
// one meaning had two spellings and the trees disagreed — surfaced by the
// canonical-printer round-trip harness, ruled illegal 2026-07-02.
//
// Canonical form: `~foo => done`.
// ============================================================================

~event foo {}
| done
| other i32

~foo => done {}

~foo()
| done |> _
| other _ |> _