feat: add linear-vesting recurring disbursement path with cliff and c…

[akprinciple]

closes #450

Summary

This PR implements the LinearVesting disbursement model for recurring schedules in the contract.

When a recurring schedule is initialized with RecurringKind::LinearVesting, disburse_recurring dynamically computes the claimable payout

Core Constraints & Math

• Cliff Gating: Ensures zero tokens are claimable prior to reaching cliff_time.
• Cap Enforcement: Clamps the cumulative claimable amount to the specified total_cap.
• Delta Payouts: Subtracts previously disbursed totals so recipients only receive the newly vested delta for the current execution window.
• Overflow Protection: Implements checked arithmetic across all elapsed time and vesting calculations to prevent integer overflow and underflow vulnerabilities.

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What Changed

• Vesting Logic Integration: Added the linear-vesting payout calculation branch inside disburse_recurring.
• Cliff Validation: Enforced cliff-period validation prior to payout evaluation.
• Accrual Tracking: Computed newly claimable token amounts relative to historical disbursements.
• Cap Clamping: Bound final disbursement amounts to the configured vesting ceiling.
• Backward Compatibility: Preserved the standard fixed-period disbursement execution path for non-vesting recurring schedules without regression.

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