Validate typed publications before encoding

Validate typed publications before encoding

Builders share payload validation with parsers without encoding and
parsing a publication for validation. Batch members use the shared
metadata and value checks without envelope encoding. Access flag
presence is a JSON input check, separate from typed descriptor checks.
Nil raw values still encode as null without changing caller data, and
the attribution limit measures the text that JSON encodes.

Issue: 8fa2f25