refactor(nns): More strictly represent neuron visibility. #25157

Job	Run time
Bazel Build All Config Check	0s
Bazel Build Fuzzers	1m 25s
Bazel Build Fuzzers AFL	1m 25s
Bazel Test All	12m 24s
Bazel Test macOS Intel	0s
Build IC	5m 26s
Cargo Build Release Linux	8m 5s
Cargo Clippy Linux	3m 38s
Python CI Tests	1m 40s
Build Determinism (//publish/binaries:upload, release, build-ic/release, false)	20s
Build Determinism (//publish/canisters:upload, canisters, build-ic/canisters, false)	14s
Build Determinism (//ic-os/guestos/envs/prod:upload_disk-img, guest-os/update-img, build-ic/icos/...	16s
Build Determinism (//ic-os/hostos/envs/prod:upload_update-img, host-os/update-img, build-ic/icos/...	15s
Build Determinism (//ic-os/setupos/envs/prod:upload_disk-img, setup-os/disk-img, build-ic/icos/se...	15s
	35m 23s

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