[P2-E4.3] Single rate-computation function (Phase-3 socket)

**Context.** Phase 3 multiplies tiers and three discount curves into pricing. Phase 2
concentrates all rate math in one pure Daml function (base rate → factors → effective
rate) so Phase 3 changes one function and its inputs, not N call sites.

**Deliverable.** `computeTrafficRate` (or equivalent) as the only place effective rate is
derived; buy path and any UI/Scan estimation call it; documented as the Phase-3 extension
point.

**Acceptance.** Grep-level: exactly one rate derivation; unit tests over factor
composition; property: factor 1.0 reproduces current gsync pricing bit-for-bit.

**Depends on.** P2-E4.2.

**Phase-3 foundation.** Tier and curve factors compose here (FR-11, FR-14..FR-22).

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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)

Read docs/planning/phase-2-epics/P2-E4-governance-discount.md and resolve the P2-E4.2 dependency first. Trace rate derivation in the buy path and UI/Scan estimation, then use grep to verify one computation entry point. Done means factor-composition unit tests pass and factor 1.0 matches current gsync pricing bit-for-bit.

Written by the indexing model from the issue text.