VisibilitySources holds Signature (channel, strength, item); the eight direct writes
go through the one writer; Visibility stays the sum; SignatureOn(channel) splits it.
VisibilityData gains Key(3) Channel. No number changes. Not yet built or tested at this
commit (verification follows).
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
