[P3-E7] Outage settlement

Implementation of FR-3's negative-balance settlement on reconnection, consuming P2-E6
consumption data — contingent on the ledger-team's Phase-1 delivery of negative traffic
balances. Leaves: settlement pricing (discounted per FR-3), grace-period config,
reconciliation-trigger behavior across outage windows. Refs: FR-3.

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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 FR-3, then confirm the P2-E6 consumption data and the ledger team's Phase-1 negative traffic balances. Done means the Phase 3 design settles the listed leaves: settlement pricing, grace-period configuration, and reconciliation-trigger behavior across outage windows, after which leaf issues can be cut.

Written by the indexing model from the issue text.