[P3-E1] Transaction-class characterization

Purchases (and eventually consumption reports) carry a declared class: composed /
app-internal / org-internal. Leaves: class declaration on the purchase path (appends to
P2-E2.3 fields); SV verification approach (detectability model, FR-28); class defaults
and dispute/reclassification governance. Refs: FR-11, FR-28.

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Milestone: Phase 3 (design-level; leaf issues cut when the P3 design settles) · Plan: [phase-3-epics.md](https://github.com/ChainSafe/canton-extending-mainnet/blob/main/docs/planning/phase-3-epics.md)

Start with docs/planning/phase-3-epics.md and the references to FR-11 and FR-28. Clarify the purchase-path class declaration, SV detectability model, defaults, and dispute or reclassification governance before implementation work begins. Done means the Phase 3 design is settled and its concrete leaf issues can be cut.

Written by the indexing model from the issue text.