evalv3: disjunction behavior / disjunction is introduced by evalv3

[mccurdyc]

cue version
cue version v0.13.0-alpha.3

go version go1.24.1
      -buildmode exe
       -compiler gc
       -trimpath true
     CGO_ENABLED 0
          GOARCH amd64
            GOOS linux
         GOAMD64 v1
cue.lang.version v0.13.0

old evaluator

CUE_EXPERIMENT=evalv3=0 cue eval -e out minimal.cue
{
    b: value: "true"
} | {
    b: {
        value: "true"
    }
}

evalv3

CUE_EXPERIMENT=evalv3=1 cue eval -e out minimal.cue
{} | {
    b: {
        value: "true"
    }
}

If you have the following scenario:

1. a disjoint (#oneof https://cuetorials.com/patterns/fields/#oneof) where option "b" is NOT just a string --- string seems to work.
2. there is a conditional involved

// minimal.cue
package playground

out: {a?: string} | {b?: {value: string}}
out: {
	if true == true {
		b: {value: "true"}
	}
}

[mccurdyc]

Above is the minimal case, but this is closer to the actual behavior we want / have in our code

// minimal.cue
package playground

#Output: {a?: string} | {b?: {value: string}}
out: #Output
out: {
	if true == true {
		b: {value: "true"}
	}
}

CUE_EXPERIMENT=evalv3=0 cue eval -e out minimal.cue
b: {
    value: "true"
}

CUE_EXPERIMENT=evalv3=1 cue eval -e out minimal.cue
{} | {
    b: {
        value: "true"
    }
}