[P2-E2.3] Purchase pricing record (recomputability)

**Context.** FR-9: every purchase must be recomputable by third parties — what price,
which discounts, what effective rate. In Phase 2 the values are near-trivial (gsync base
rate, at most the governance discount) but the *fields* must exist now so Phase-3 tiers
and curves change values, not schema.

**Deliverable.** Purchase records carry appended fields: effective USD/MB rate applied,
discount factor applied (1.0 default), and the round's CC/USD conversion used. Decide and
document: fields on `MemberTraffic` vs a purchase-record sibling (weigh merge semantics —
merged records must aggregate or list per-purchase pricing).

**Acceptance.** For any purchase, a third party can recompute burned CC from recorded
fields + public config; merge behavior for pricing fields defined and tested.

**Depends on.** P2-E2.1; feeds P2-E4.2 (discount records into these fields), P2-E7.2.

**Phase-3 foundation.** Tier id and per-discount factors append here later; reward
eligibility flags (FR-25) attach to the same record.

**Refs.** FR-9.

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Epic: #65 · Plan: [P2-E2-traffic-purchase.md](https://github.com/ChainSafe/canton-extending-mainnet/blob/main/docs/planning/phase-2-epics/P2-E2-traffic-purchase.md)