Add yield boosters for long-term staking

[nanaf6203-bit]

Problem Statement

No lock-up incentives in the staking contract.

Why it matters

User behaviour + capital lockup.

Technical Context

Add lockup_period: u64 + boost_curve.

Expected Outcome

Longer stake → higher reward weight.

Acceptance Criteria

1. Test vector table for various locks.
2. Kani harness.
3. SDK + docs.

Implementation Notes

New logic.

Files or modules likely to be affected

contracts/staking/src/lib.rs.

Dependencies

None.

Difficulty level

MEDIUM.

Estimated effort

M (~3 days).