[P2-E2] CC-funded traffic purchase

Traffic on a dedicated synchronizer is funded by burning CC on the global synchronizer via
the existing purchase flow: `AmuletRules_BuyMemberTraffic` names the synchronizer id and
member, burns CC, and creates/merges a `MemberTraffic` record. This epic extends that flow
to registered dedicated synchronizers at unchanged (gsync) rates — the must-burn invariant
(FR-4): a participant's traffic balance on a dedicated synchronizer increases only via CC
burned on mainnet for that participant on that synchronizer. All changes are appended
`Optional`s (SCU-safe).

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Milestone: Phase 2 · Plan: [P2-E2-traffic-purchase.md](https://github.com/ChainSafe/canton-extending-mainnet/blob/main/docs/planning/phase-2-epics/P2-E2-traffic-purchase.md) · Leaves filed as sub-issues.

Start with docs/planning/phase-2-epics/P2-E2-traffic-purchase.md and the existing AmuletRules_BuyMemberTraffic purchase flow. The completed work should support registered dedicated synchronizers at unchanged gsync rates, preserve the must-burn invariant, and append only Optional fields for SCU safety.

Written by the indexing model from the issue text.