[E9-1] Staking, tracking, duration, draw-down penalty + shortfall burn

[salindne]

Net-new commitment contracts (stake + duration + penalty) and the coupon-free shortfall burn. None exists on-ledger today.

Acceptance criteria:

• A committed buyer's stake, duration, and under-consumption penalty are modeled and enforced

Epic: #12

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