feat(staking): implement delegated staking (#246)

[FaveTeamz]

feat(staking): implement delegated staking (#246)

• Add ValidatorInfo, DelegationRecord, DeactivationReason types and constants
• Add 9 new error variants (InsufficientValidatorStake, AlreadyDelegated, etc.)
• Add delegation storage fields, 10 events, 12 new messages to staking contract
• Implement per-validator accumulator reward model (O(1) per delegator claim)
• Implement slash propagation to all active delegators via secondary index
• Add 30 unit tests covering all delegation paths and end-to-end flows

closes #246

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[nanaf6203-bit]

LGTM