084 reject when only required terminal branch

// ============================================================================
// Test 210_084: MUST_FAIL — required terminal branch covered only by a
//                          `when`-guarded handler is incomplete coverage.
//
// `when` guards NARROW a handler — they pick out a subset of fires that
// match the predicate. They do NOT satisfy the coverage obligation for
// a required branch by themselves. If the guard's false case has no
// handler, a fire of the required branch can silently produce no work,
// which is a coverage hole that looks like coverage in source.
//
// Rule: a required branch with only `when`-guarded handlers is incomplete.
// Rescue shapes:
//   1. Add an unguarded sibling handler: `| ok x when x > 0 |> a`
//                                         `| ok _ |> b`
//   2. Add a metatype catch-all that engages: `|? Transition t |> log(t)`.
//
// Optionals do NOT have this requirement (see 210_086) because "no handler
// for this fire" is a legal trajectory under the optional contract.
//
// Expected: validator rejects the flow because `| ok` is required but
// only covered by a guarded handler.
// ============================================================================

~event compute { x: i32 }
| ok i32
| err []const u8

~proc compute|zig {
    if (x < 0) {
        return .{ .err = "negative" };
    }
    return .{ .ok = x * 2 };
}

~compute(x: 5)
| ok v when v > 0 |> _
| err _ |> _