[P2-E4.2] Buy path honors the discount

**Context.** The buy computes CC to burn from USD price × discount factor at the round's
conversion. The applied factor and effective rate are recorded on the purchase (P2-E2.3)
so the burn is recomputable.

**Deliverable.** Gate reads the discount (disclosed alongside registration + fee state),
applies it in rate computation, records factor + effective rate.

**Acceptance.** Daml Script: discounted buy burns proportionally less CC; recorded fields
recompute exactly; discount for sync A never applies to sync B; no discount → identical
to gsync rate.

**Depends on.** P2-E4.1, P2-E2.1, P2-E2.3.

**Refs.** FR-9, FR-13.

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Epic: #67 · Plan: [P2-E4-governance-discount.md](https://github.com/ChainSafe/canton-extending-mainnet/blob/main/docs/planning/phase-2-epics/P2-E4-governance-discount.md)