Metadata attached to one user message content item.
Each branch carries its own discriminator. The common semantic handoff stays
in AutoBeUserMessageContentMetadata.ISemantic, the persisted record
shape starts at AutoBeUserMessageContentMetadata.IBase, and
branch-specific facts live only on the branch that can justify them. The
enclosing user message content item owns the stable identifier.
Metadata attached to one user message content item.
Each branch carries its own discriminator. The common semantic handoff stays in AutoBeUserMessageContentMetadata.ISemantic, the persisted record shape starts at AutoBeUserMessageContentMetadata.IBase, and branch-specific facts live only on the branch that can justify them. The enclosing user message content item owns the stable identifier.