[E8-2] Enforce the discount curve at burn time

[salindne]

Feed the CIP discount curve (tiered mode; the (1 - D) Section-6.2 correction) into the buy choice / computeSynchronizerFees, replacing the flat byte-linear price. Add the governance-voted fixed discount. Reference (parked): the intended tiered math already exists as validated Daml in sync-pricing/daml/SyncPricing.daml.

Key files: sync-pricing/daml/SyncPricing.daml (priceCents, DiscountMode.Tiered)

Acceptance criteria:

• On-ledger price for a class/throughput/duration matches the shadow engine's tiered output

Depends on: E7-1, E8-1

Epic: ChainSafe/canton-extending-mainnet#20

Source: docs/planning/extending-mainnet-work-plan.md