Rewrite canonical IB slice in current Idriç style

[isomorphisms]

Stacked on #47 so the ongoing canonical-model fixes can continue independently.

This pass follows the current Idriç source-style authority from Idric PR #76 while building with the current Idriç default head d2463ec8a3a0dd4ac167029927452f3e83805dc3.

Changes:

• replace implementation-oriented IB.Information.Model with IB.Strand;
• replace IB.Prepaint.Model with IB.Prepaint;
• remove Nat from new IB domain-facing source;
• generate zero-capable revision sequences through first_revision / next_revision;
• keep structural references nonempty behind checked constructors;
• use the closed strand block variants Heading/Text/Link/TableRow/Form/Image;
• update the version-1 serializer and smoke acceptance to the new semantic surface;
• retain the old IB.Information extractor for a later migration rather than mixing parser rewrite into this type pass.

The existing inherited IB.Information file is still implementation-first and remains the next migration target after this slice compiles cleanly.