[P3-E4] Commitments & staking

Committed traffic rates backed by a staked bond (reuses the P2-E3.1 state pattern).
Leaves: commitment contract (rate, duration, bond); shortfall handling (bond draws as
forced purchases of committed traffic); termination (remaining bond burns, no credits);
bond top-up and return; interaction with discounts (duration discount presumes
commitment). Refs: FR-18..FR-22 (note: FR-18/19 numbering duplicated in the source
table — cite by text until fixed).

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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 review FR-18 through FR-22, using the requirement text because the source table has duplicated numbering. Define the commitment contract, shortfall handling, termination, bond top-up and return, and discount interaction at the Phase 3 design level. Done means the design is settled and the corresponding leaf issues can be cut.

Written by the indexing model from the issue text.