Define canonical Idriç information and prepaint model

[isomorphisms]

Establish the first canonical information/prepaint type boundary without replacing the existing extractor yet.

• source-backed Strand
• reserve Fragment for a possible later smaller unit extracted from a downloaded/source document
• six-case HeadingLevel
• closed information block sum including images
• structurally nonempty table rows and nonempty source/image/address references
• distinct zero-capable RevisionSequence and partial/complete prepaint state
• version-1 serializer with increasing-revision validation
• deterministic smoke acceptance using the currently pinned Idriç compiler

The existing IB.Information extractor remains intact in this first slice so behavior can be migrated separately after the model is proven.