[P3-E2] Tiered per-synchronizer pricing

The 200/150/10¢ tier structure enters the rate pipeline. Leaves: tier schedule as
governed config (per-class USD rates, gsync pinned 100¢ — FR-12); rate function extension
(P2-E4.3 socket); per-synchronizer pricing schema if per-sync overrides survive design;
recomputability proof extended to tiers (FR-9). Refs: FR-11, FR-12.

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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-9, FR-11, and FR-12. Review the P2-E4.3 rate-function socket and determine how the tier schedule, gsync rate, per-synchronizer overrides, and recomputability requirements fit together. Done means the Phase 3 design is settled well enough for concrete leaf issues to be cut.

Written by the indexing model from the issue text.