[P3-E3] Discount curves

Throughput, duration, and utility discounts, multiplicative on the tier rate. Leaves:
curve definitions as governed config; measurement inputs (throughput needs consumption
history — P2-E6 data); curve application in the rate function; anti-gaming bounds
(caps, floors, FR-14..FR-17). Refs: FR-14..FR-17.

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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 definitions of FR-14 through FR-17. Review the dependencies on governed curve configuration and P2-E6 consumption history, then clarify how discounts enter the rate function and how caps and floors prevent gaming. Done means the Phase 3 design is settled and concrete leaf issues can be cut.

Written by the indexing model from the issue text.