[P2-E4] Per-synchronizer governance discount

The single Phase-2 pricing lever (FR-13): Super Validators can vote a discount on the
traffic price for a specific dedicated synchronizer — the governance bridge ahead of the
Phase-3 pricing model. No curves, no tiers, no operator self-service: one voted factor
per synchronizer.

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Milestone: Phase 2 · 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) · Leaves filed as sub-issues.

Start by reading docs/planning/phase-2-epics/P2-E4-governance-discount.md, then follow the sub-issues it identifies to locate the implementation entry points. Use that plan and the sub-issue acceptance criteria to determine what must be delivered: a governance-voted discount factor for each dedicated synchronizer, without curves, tiers, or operator self-service.

Written by the indexing model from the issue text.