pkg/list: UniqueItems validation breaks when used on a hidden list of structs next to an embedded value

[jpluscplusm]

What version of CUE are you using (cue version)?

$ cue version
cue version v0.12.0-alpha.1.0.20250103115825-74a0c9d01e05

go version go1.23.0
      -buildmode exe
       -compiler gc
  DefaultGODEBUG asynctimerchan=1,gotypesalias=0,httpservecontentkeepheaders=1,tls3des=1,tlskyber=0,x509keypairleaf=0,x509negativeserial=1
     CGO_ENABLED 1
          GOARCH amd64
            GOOS linux
         GOAMD64 v1
cue.lang.version v0.12.0

Does this issue reproduce with the latest stable release?

No; the repro passes with v0.11.1 (modulo a slightly different validation error message).

What did you do?

! exec cue vet -c working.cue
cmp stderr out.working

! exec cue vet -c broken.cue

-- working.cue --
import "list"

A: [1, 1]
B: {
	42
	_check: list.UniqueItems & A
}
-- broken.cue --
import "list"

A: [{a: 1}, {a: 1}]
B: {
	42
	_check: list.UniqueItems & A
}
-- out.working --
B._check: invalid value [1,1] (does not satisfy list.UniqueItems): equal value (1) at position 0 and 1:
    ./working.cue:6:10

What did you expect to see?

A passing test.

What did you see instead?

> ! exec cue vet -c working.cue
[stderr]
B._check: invalid value [1,1] (does not satisfy list.UniqueItems): equal value (1) at position 0 and 1:
    ./working.cue:6:10
[exit status 1]
> cmp stderr out.working
> ! exec cue vet -c broken.cue
FAIL: /tmp/testscript4225105853/list.uniqueitems.txtar/script.txtar:4: unexpected command success

[jpluscplusm]

This form is being used as the basis of a rewrite of some cuelang.org content. This regression (post https://cuelang.org/cl/1204378) is blocking https://cuelang.org/cl/1206577.